This is bizzare. Are these enemies shooting anything at all? How fast are they? I encountered something like this bug once in testing, but I thought it was fixed. Which wave did this start occurring on?
It started from the first few waves like wave 5, they appear to be shooting something as I see the visual effect of shooting (like that pew) but the expected projectile is invisible and does no damage, they are at normal speed ie faster than those round scatter shot enemies and the fireball explosion enemies, but slower than the self destruct enemies. Sometimes the bug happens sometimes is doesn't , it depends. I tried loading the same file for one wave for 10 times and 3-5 had invisibles.