Will try to explain the situation. It is 100% reproductable.
For example I have a sniper with two granades in the slot. I see the alien. Want to kill 'em using the granade.
Click grandae, yellow box appears around the granade, ok.
Now i want to change my decision and want to use the sniper rifle insteard of grande.
case1:
Now I click the Sniper rifle and yellow box appears around it as well.. when moving mouse cursor to the alien, it shows that I use the granade, but not the rifle.. so cannot make a sniper shot.
case2:
Now I click the granade button again to disable the granade selection, but the yellow box around the grandade does not desepear, so granade become selected forever, can rethink the selection. So I have to chose another soldier.