Thank you both for the suggestions. I tried changing the tooltip property, as well as adding a new form (both the classic and the modern form). With the new forms, I also tried the same data source as well as different data sources, and both gave the same result.
mmbr1606, I must have not had the app refreshed when I tested one of your suggestions earlier. I did find that setting the value field of the text box to "" removed the "Text box" text. This does appear to be the issue - when the value of the data card value is set to Parent.Default and the field is empty in the data source (or when the form mode is "New"), it will show "Text box" as the data card value.
This only started happening ~2 weeks ago, and prior to that time blank values would show correctly. Any thoughts?