I have solved my problem. For anyone who may be curious, here is the solution. It's very simple - I had to update Survey123 Connect to the newest version and then republish the survey. Then, survey users had to DELETE and download the survey again; simply refreshing the survey without deleting it was not effective.
What happened was that we found out at the 2020 UC that in version 3.10 of S123, the "extensions" folder had to be named "scripts." We have a custom javascript function in this folder for this survey. So a few months ago, we changed the folder name and republished the survey. We had users refresh the survey (rather than fully deleting it and redownloading it). But we did not notice that we were using version 3.5 of Survey123 Connect. (On that note, I have posted and commented in a couple of "Ideas" in the Esri Community to have the app actually give you some indication that a new version is available!!! That would have prevented this whole big ordeal that took several staff members multiple weeks to figure out.)
So, version 3.5 of Survey123 Connect did not publish our scripts folder. Anyone who newly downloaded the survey after that date did not get the scripts folder, or actually any of the other changes we made to the survey. That is why this one user had the problem - she was the only one to download the survey (rather than refresh an existing survey) during this time. Actually, there was also another user who also newly downloaded the survey, and he had the same problem, but he never told us about it, so we didn't know about that until after a ton of digging.
The other users, who previously had the survey and merely refreshed it after we did our survey edits, actually retained the extensions folder with the javascript file, so that file continued to work for them, weirdly.
So our solution was to update Survey123 Connect, republish the survey, and ask survey users to DELETE and redownload the survey.