It’s not exactly stuck at 100%. The issue here is the progress bar is estimating how close you are to finishing and rounds up. So if you’re 99.6% done with the kill count, it’ll show up as 100% even though you aren’t finished yet. Check the quest using a third party app such as Ishtar Commander or DIM (Destiny Item Manager) to get a solid number on the number of kills you have left to get.