Quest capped out at 7, but requires 8 couldn’t complete. Abandoning quest and picking back up to see if that fixes it.
EDIT:
dropping and picking quest back up fixed.
I was able to collect eight crabs. However, I am unable to turn in the quest (quest giver does not have a question mark and I do not have the crabs in my bag) and the quest itself goes from showing 0/8 on the quest log to just showing the quest name. When I open it up, the dialog box says I have completed the quest.
I tried dropping and picking quest up again, but it did not help.
Still an issue as of 7/22/19. Had to abandon and re-acquire.
Today I also collected 7/8 creatures (a mix of crabs and muck creepers though) and then was unable to see any more or use the quest item on any more creatures. Abandoned and was able to re-complete it within 2 mins the second time around.
I think it might have been related to me capturing the #8 creature while mounted? Because at first my quest UI said I was done so I flew around doing other things and when I went back to turn it in, it was back to showing 7/8 on the quest progress. Doing /reload did not work.