It didn’t for me.
Saving to main menu and then returning fixed it for me (after I ended my turn prematurely and let the brumak have a turn). I know that hasnt worked for everyone. Ironman playthrough for me if that makes any difference.
This worked for me. Tried mouseclicking, tab, shift, reload, starting the mission from scratch and none of those worked
I had the same bug, At the start of the encounter I could TAB between all my units without a problem. When I got to the 1/3 and 2/3 Checkpoints the TAB button just stopped working. To fix this issue, I had to save & quit to Desktop and reopen Gears Tactics to allow me to TAB again.
Hopefully this can be of some assistance in fixing this bug.