The issue is that when you opened the day, you had one or more windows open.

It corrupted the day.

Do you mean windows from another race that day? I have only looked at this once race since I restarted everything.
In any case, how do I fix it?

It can actually be the same day.
You MUST have all windows closed when you open a day - even if it's the same day.

Having an open window stops the old data from being deleted because the files are locked by the window.

How you fix it...
Reimport the day from scratch, including results if there were any.

I went back to the Import tool, Imported PPs & Charts for this day, Posted Key Data, opened deT, HQ, brought up 2 other races from that day and they were fine in AI Assistant, killed them, brought up the one race in question - and the problem is still there. I have done this process a couple of times now.

Did you remember to reopen the day?

I can meet right now.

ONE-ON-ONE
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I will wait in the meeting until 9:00am pacific time.

@Dave Schwartz Day job interfered

I have seen something similar (maybe identical) and I solved it by opening the race following the one you are having the problem with, then closing it and then re-opening the problem race.

I have seen something similar (maybe identical) and I solved it by opening the race following the one you are having the problem with, then closing it and then re-opening the problem race. — Red Knave

That worked! Thanks Bob!

For the record it was the next race after at the same track (TAM 02) as the problem race (TAM 01) in my case - it did not work just to open the next race in the post time sequence.

I wonder if the underlying data has a missing end-of-line marker or some other anomaly.

These are all Clarion files - not text files.

Best guess is that the fix you suggested is a mitigation of the underlying problem I described.

It's caused by DUPLICATES of a key field as they load into one of several files.

Right, I forgot that. Anyway, it does allow moving forward.

Agree completely.
Thank you for the great advice.