I have just tried to replicate the problem.
If I DRAG a concealment counter on to a Chicom unit and then remove it either by (a) right click -> delete or (b) dragging it off, I see the underlying unit(s) with no small ?
If I use right click -> conceal to add a concealment counter to a Chicom stack, then only the selected unit gets a ? counter. So that's a problem. When I remove the concealment counter by dragging it off the stack, the ? unit loses the small ?. However if I remove the concealment counter by right-click -> delete then the ? unit retains its small ?. Which is another problem (if I subsequently right-click -> conceal on the unit with the small ?, the small ? disappears).
In short, there is a bug in the behaviour of the Chicom ? counter. But there are ways of working around it in the short term unit the bug is fixed.
I will add the bug to the bug list. I don't expect it to be fixed in the official release of vasl6.6.3 which will come out before the end of October.