Of course they test it, and because they release beta versions to the public, many private users test it as well.
People use their Macs in different ways, including Apple’s developers. There are two issues I’m now aware of with Mission Control (lagging and moving groups of windows), and I did not notice either of them before user posts here. I’m a heavy user of Mission Control (5-6 Spaces and lots of switching between them), but I almost never use full screen and I don’t have my app windows grouped.
Apple prioritizes bug fixes based on report frequency, so if not many users run into this issue (as I didn’t) and report it, there may be a long wait for a fix.
“We” are users like you and have no power to implement any suggestions. Apple employees are generally precluded from even reading these forums, with the exception of moderators who only monitor for compliance with the T&C.
Bugs can be reported to Apple here:
Feedback - macOS - Apple
Note that a few users have determined that the lagging issue (which I do not see) was caused by a third party extension they had installed.
See if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.
Start up your Mac in safe mode - Apple Support
If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.
Add a user or group on Mac - Apple Support
Once the testing is complete, the new account can easily be deleted.
Delete a user or group on Mac – Apple Support
Those steps will help to isolate the problem.
It might help to download and run EtreCheck, written by a long-time forum member. The free version is fine. Post the full report using the Additional Text button. No personal information is revealed.
