So any unit in melee takes 50% less ranged damage, that explains it. I wasn't sure if this was a trait of melee in general (I thought no since none of the old wikis mention it) or if it was an unintentional effect of melee resistance. It definitely happens though, it's VERY noticeable. Not any unit...

I thought this had been fixed by adding a common cooldown to every ammo type. Do you have a replay featuring this? Yes, attaching it. It happens around the 19 min mark. Also since the sarge is apparently the last to die i'm obviously wrong about the trigger. For some reason I just assumed the next ...

This might have been mentioned but there's a nasty glitch with sternguard getting stuck in ammo changing state. The trigger seems to be when the sarge is killed during the ammo switch. Once locked in this state (showing the up arrow icon) only ATSKNF can be used and squad cannot fire their bolters.