Nothing happened affer elder/guardian kill

Sometimes when i kill elder or his guardian, they didn't drop anything.
Then when i leave the map ,they are still in altas(but they are no longer in the map,ofc they were dead) ,which means i have to buy more maps
This happened to me 5 times in total (2 elder,3 guardian ) , unbelievable.
What amazed me is that this could happen in a row, T13 eradicator in desert spring 2 times followed by T15 enslaver in basilica.
I don't understand how such bug is allowed to even exist, not to mention it happens so frequently.