Very Strange List Import Behavior

I just experienced very strange behavior when trying to import a CSV file with 23.7K records into a static list.

1. First try: All but 272 records were successfully imported. There was no error message so at first I thought my original must have had dupes but I checked and there were none.
2. So I tried importing the full list again into the same static list with the same result.
3. I created 2nd static list and imported a CSV file containing only the 272 missing records. This time - again - some records were missing (I didn't write it down but I think 264 came in).
4. I tried importing the same 272-record file into the same (2nd) static list. This time - the number 272 flashed for a moment on the screen but then it reverted to zero. And the end result was that the list which has had roughly 260 people in it prior to the import was empty.

Has anyone ever seen anything like this before? I've been working in Marketo for 9 years and am baffled.

UPDATE: I made a new copy of my list of 272 records in case the file had gotten corrupted in some way that was tripping Marketo up. I did this because you know how sometimes in the field mapping section of the list import Marketo sees a duplicate column where there is none? This was happening. It happened with the new list, too but this time all 272 records made it into my list. However, a few moments later I looked again and the list was empty.