@BCBuizer - I created that column just before I made the power app and have not changed the name. Investigating why this might be happening, I discovered that OData can be used with SharePoint. (I don't normally work in this area, so it's all foreign to me.) Why they are connected in my case is unknown. Our central IT department deploys back-end settings across the organization that I have no knowledge of, but could be influencing this.
My SharePoint list was copied and adapted from a large, 35-column list that I think was originally exported from InfoPath years ago, by people who no longer work here. Maybe something weird happened with the export, or whomever finalized the original SharePoint list set things up in a way none of us can figure out. My column was brand new, though. I've asked my IT unit to investigate but so far, no progress.
(Another example of weirdness: I recently noticed the Locations Choice column in SharePoint was set to checkboxes, which allows multi-selections. Since we don't want multiple Locations, I changed it to single choice. But then the app started inconsistently writing to that list field. (The app's combobox was set to NOT allow multiples.) So I had to go into the SP list, turn checkboxes (multiples) back on, and writing to the list became consistent. Clearly, changing settings long after your list and app are operating together can cause all kinds of instability. I haven't changed anything with that Backup contact field, but am making a lot of changes and additions to the app even though it's live. Building the airplane while we are flying it.)