Students using 1.20.81 unable to join each other's worlds

I am teaching a Minecraft-based class to a group of students using Minecraft PE (not education edition). As of about three weeks ago, they can no longer join each other's worlds. Do you know of a change that may be causing this to happen?

For several years before this we have not experienced this problem. It did start prior to all students being on the current version of Minecraft, but it's still happening after updating all the devices. There are about 5-7% of the devices that are able to join worlds, but it seems like a random occurrence.

Also, many of the devices freeze upon loading the start screen. If they tap "Play" within about 1 second, it will load the world's screen. If they take longer than about a second, it freezes and they need to restart the app.

Any help will be greatly appreciated!

Thanks,

Paul

:)