It was the encryption! I decrypted both tilda and non-tilda files, opened the tilda file from within TT, saved-as and overwrote the old non-tilda file, exited TT. I added a z prefix to the tilda file to get it out of the picture. I reopened the "new" non-tilda file, exited TT saving the file, and verified that no new tilda files were lingering and the non-tilda file mod date incremented.
This is a short-term fix. I did not have this problem last year using the same encryption software, Boxcryptor. I'm uncomfortable having my tax file unencrypted. The only apparent difference on my end is that last year I was using Google Drive and this year I'm on OneDrive.
