Invincible mod during the fight

i witness this bug a few time in a row(relaunch game make it disappear)

during the siege some of mob can firstly spawn invincible. and then disappear(become invisible), but they still can damage you, and cause of them the siege doesnt end, so your only way to get out is either die or teleport away, but this make dungeons(for me invincible mob appears only there) impossible to complete

video just refuse to upload, so sorry, but i have a youtube link