When you set the same confirmation page for two separate forms, it can cause one of the forms to show the wrong confirmation and fail to record entries.
I discovered the issue because the form was giving confirmation text instead of going to the confirmation page, as set up (and double checked) in confirmation settings.
Suspicious and never having done this before, I changed the confirmation page for the other form, and immediately the issue was resolved. The other form now goes to the proper confirmation page, and the entries are being recorded again.
What I’m concerned about is if this is a normal and known issue or if it is indicative of some deeper issue in my setup – I obviously never want entries to go missing again. Anything else I should be checking?