Last year I had an HNL-IAH return segment not post automatically.
It was the last of 6 segments and the only one to not post.

After a couple of conversations with the MP desk they manually posted it (while I waited) but had to post it on the day before I actually flew the segment.

Speculating, there may be some back end issue where something gets bollocked up and perhaps they can't delete the bad segment and also can't enter the same flight segment twice on the same day. So they do a manual post of the segment on a different day. Doesn't seem to hurt for annual qualification; the PQM miles still accrued.

I'm sure they'd post it in your favor by manually entering the accrual on 12/30 rather than 1/1 if it happened on 12/31. Otherwise, as above, I wouldn't worry about it.