Spent some time just watching FT8 waterfalls, where it is obvious that some stations are off timing.
What is telling that quite a few seem to be on the same “off” together, suggesting that there may be a computer clock induced issue.

Ragged Finish to an FT8 Cycle

Ragged Start to an FT8 Cycle
My theory is that many stations experienced the same NTP issue simultaneously moves their time sync off in-mass.
If the cumulative offset is too much, decoding stops and the operator will look for a fix, otherwise they will continue on.
There are specifications to all this, but as I mentioned upgrading my NTP service to the latest sorted out my PCs.
73
Steve
K9ZW