I’m having a problem joining delta and beta preview update ring

Seems you have to be invited to join the Alpha ring

https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

The current Xbox One Update Preview rings are:

• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window.
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
• Omega – Open to anyone and receives system updates shortly before the GA (General Availability) release.

Hi Volcannon,

Yes, you can rejoin after leaving at any point. It's possible that by joining and gaining exp so that you move up the preview rings to Delta at least so that you may get a future preview update where this issue is addressed and you can test it out for yourself
before those in the Omega ring or not in the preview program. Beta ring previews updates before Delta ring and the Alpha ring, where updates are tested first, requires an invitation to join.

Either way a solution to this will more than likely be tested in the preview program first. News regarding what is included in recent preview updates for the different ring levels can be found here:
https://news.xbox.com/en-us/insider/

Also here is an FAQ all about the Xbox Insider Program:
https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

In my understanding the Dashboard experiment, A, B, C selection, is totally random.

As "Both consoles say their OS Version is (19h1_release_xbox_dev_1911.191028-1335)" your consoles are either in the Beta or Delta ring.

Both those rings and Omega ring are getting a preview update News release notes Beta Delta Omega
Insiders today, which you can manually start or wait until it becomes mandatory.

After you do the update, if you want to change rings, it will be the perfect time to change as all those rings will be on the same release. Subject to eligibility.

You just have to continue to be active in the program, take part in surveys, etc. but Alpha is also by invite only so you would have to wait to be invited into it by Microsoft.

How do the Xbox One Update Preview rings differ and how do I change rings?

There are four Xbox One Update Preview rings, which allow you control when and how often you will get early access to console updates. You’ll gain access to rings based on your activity as an Xbox Insider. If you’ve been invited to more than one ring, you
can switch between them from the Xbox Insider Hub.

The current Xbox One Update Preview rings are:

• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window.
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
• Omega – Open to anyone and receives system updates shortly before the GA (General Availability) release.

The decision to join the latest iteration of XBox Insiders, 18 months ago, was made to solve some issues I was having when I purchased
a XBox X and couldn't find any satisfactory feedback.

Joining gave me the resources to Report a problem using the Insider Hub which I have used many, many times initially
in the Omega, then Delta, and now Beta Rings.

The Reddit XBox Insiders Forum and the Reddit XBox
Insider wiki for all things Insider related is also a great resource to help with issues and to provide information on upcoming update releases to the XBox console operating system.

Deciding what ring to join depends on what level of involvement you want to have with the program.

The current Xbox One Update Preview rings are:

• Alpha Skip-Ahead – Open on an “invite only” basis and receives updates as part of a different future branch. This ring receives updates far ahead of General Availability (GA), with features that may not exist in the other rings.
• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window for the current development branch.
  
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
  
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
  
• Omega – Open to anyone and receives system updates shortly before the GA release.

I'm still waiting for an invite but quite happy with Beta.