@Pat Hartman - See Replies #13, #17, and #18.
@MajP knew exactly what I was talking about and got me working. Somewhat agreed, UserForms are not part of Access, and they work differently, and they often work differently in Access than they do in Excel or Word.
I likely got much better assistance here than I would have posting the question on an Excel forum - although I did link to replies on StackOverflow that pointed me in the correct direction.
The userform in question was a replacement for the InputBox control. It was somewhat inspired by Enhanced Message Box -
https://datenbank-projekt.de/projekte/improved-enhanced-message-box-ms-access - but he DOES use a standard Access Form.
What it does that the standard input box doesn't do:
I have it set to automatically scale to fit any text that I put on the box.
I can have up to three combo boxes on it, and can show or hide any of them.
Each combobox can be configured as either a blank input box or a drop-down, and I can choose whether the drop down is mandatory, or whether other values can be manually entered.
I haven't tested it, but it likely would work the same way in Excel or Word.
Other than the last statement - yes, I probably could have done the same thing with a standard Access pop-up data form.
The short answer is that I have this userform called probably 100 or so different times in my database. It was much simpler to modify 6 of the instances to read the drop-down values from a table, than is would have been to create a standard Access data form that looked and worked identically to my userform and modify 100 callouts to call out the new form instead of the old one, and verify it worked correctly in each instance - just to make this one case able to use a query, than it was to modify the userform to work.