I'm attaching a link to a screenshot from Steam (sorry, 4k) showing part of an issue I've run into.

Basically, control group numbers eventually get confused. Sometimes this happens when control groups have gotten eradicated, and I've attempted to reuse those numbers for different formations. I'm not sure actually of all of the ways this happens, but it does.

In the image linked, you clearly see the game telling me that control group 1 is what all these units belong to, but in the minimap on the left the game thinks these units belong to 2 and 3.

Sometimes after the confusion sets in I'll hit 8, say, and it will take me to a bunch of units that are clearly marked as control group 5. In a previous game the game thought that both 8 and 9 applied to the same group of units.

I recognize there is some overlap here because a control group can apparently be applied to more than one Army. At least, I've had multiple dreadnoughts that I grouped together in a control group even though they cannot be in the same army.