Yesterday I was trying to complete the Hardcore challenge (required 5005 words). I marked the last couple of them learned, got the popup notification that the target had been met, opened the challenge, saw 5006/5005, and then went to bed.
This morning I saw this:
where did they go overnight? I didn’t unmark any after that 
I realize this is an absurd thing to be concerned about since there’s no difference in the actual amount learned between 5004 and 5006 words marked learned, but the timing is pretty frustrating.