Eater of worlds boss fragment ( 28/28 maps ) wont drop with sentinel

I dont know if this is a known bug or not, but if you buff the map boss up with your stalker sentinel and or pandemonium sentinel, the boss fragment will not drop and the counter will go down to 0/28. This has happened to me 2 times in a row now ( 48 maps later ), i've gotten the first shrek boss, but not the last one.

filter hid it, not bugged.