V10.1.4588 Known bug? "Export CSV" can omit some messages!

On this folder:

image

I select all messages (but can find no confirmation of the qty to screenshot - ideas?), do “Export to CSV…”, and get a file comprising:

which has (discounting the column names header line) five fewer lines than messages!

Is this a known bug? Any workaround?

I’d like to find which messages are missing. By eye I have determined they aren’t amongst the first and last few messages, but to by eye locate check all 250K would take me a week. Any idea how to locate the missing ones?

Thanks.