My achievements aren't showing up in the newsfeed the last couple of weeks.

1. Make sure your console is connected to the internet
2. Check the Xbox status page to see if there are any known issues that could be causing this problem. You can find the Xbox status page here: https://support.xbox.com/en-US/xbox-live-status
3. Try resetting your console's network settings. To do this, follow these steps:
- Press the Xbox button.
- Move right to the System Tab.
- Scroll down and select Settings.
- Scroll down to Network.
- Select Network Settings.
- Scroll down and select Advanced Settings.
- Scroll down and select Alternate MAC Address.
- Select the Clear button.
- Then restart your console to apply the changes.
4. If none of the above steps resolve the issue, you may need to clear the cache on your console. To do this, follow these steps:
- Turn off your console by pressing the Xbox button on the front of the console for about 10 seconds, until it shuts down completely.
- Unplug the console's power cable. Wait 10 seconds.
Important Be sure to wait at least 10 seconds. This step resets the power supply.
- Plug the console power cable back in.
- Press the Xbox button on your console to turn it on.

I periodically see the same thing and have done for a long time. Unfortunately, I think the system is just buggy. I know that's not much help, but it's definitely not just you experiencing it.

Within the last week, this is game is not showing in my game list anymore.. the achievements aren't either. Would anyone know why?

I did, I tried doing that over and over again but it doesn’t work...

Another good question to ask, while being an obvious one it is still something people overlook from time to time. Are you playing Roblox through the port of it on the Xbox One or are you trying to play it on PC through something like the Roblox website,
or in this case I believe the PC version of Roblox does have its own launcher akin to Minecraft. You'd need to be on the Xbox port of the game for achievements to work, I know that sounds like common sense but you'd be surprised how that can be a mistake sometimes.