Attacking bunkers when defender does not fire back.... MINOR SPOILER FOR GERTSOVKA

[Kwacker]

Question from the Gertsovka scenario: I approach the bunker on the left that contains an AT unit with on-foot Panzer Grenadiers. The AT unit does not fire at them and I cannot assault or fire at the bunker. The game gives a message that I cannot move into an occupied hex and a no valid target message in each case. As I can't assault or fire and the unit does not fire back to become spotted, I have to bypass the bunker - which does not seem right.

What am I missing?

The same thing happens initially at the bunker at (4,6) - but at least this time the AT eventually shoots back, becomes spotted and I can assault.

Help appreciated!
Simon.