Hi, I’m completely new to the franchise and haven’t played an RTS in 20 years. I like it so far. I’m playing on the Xbox btw.
Now to the problem. Managing your troops and selecting and deselcting units is super important. You always must have control over the units you want to control. Often I select the whole screen with RB and then deselect single units with holding RT and pressing B on each unit to take them out of my groups. Today I encountered a bug that sometimes prevents me from doing that.
I have spent like 2 hours figuring out what causes this, but I couldn’t reproduce it intentionally. I tried holding RT longer before pressing B, tried different unit combinations like with a spartan or a spartan in a vehicle. I even loaded different savegames. It just comes and goes.
It seems like it happens when you have quite a lot of units. At least it didn’t occur when I had just a few units. But maybe this was a coincident.
This is annoying to the point that it drives me crazy. You have to multi task in this game so bad, manage three or four different groups, send them to attack the enemy, build your base, upgrading it, etc. And then this happens and you spent 2 minutes allocating the units to the right group without being able to react to the alerts. 
Please fix this!