Actually, it turned out that my TurboTax wasn't up-to-date even though I had clicked "Online --> Check for updates" multiple times and it was claiming that the product was up-to-date. I believed as much since I had installed it from a recently downloaded EXE file.
I was on the phone with customer service for a half hour about this problem. They had me uninstall and reinstall TurboTax and "Check for updates", but they didn't think to confirm the version number (which was 2021.47.1.18 - the version number of TurboTax.exe).
After seeing your reply, I found https://ttlc.intuit.com/community/updating/help/manually-update-turbotax-for-windows-soft[product ke...
After installing this, TurboTax is version 2021.47.21.82 and form 1116 seems to be working.
