I have been seeing errors appearing for formulas that used to work, even though neither they or the underlying table have been updated.
A probable cause of the formula error appears to stem from AppSheet incorrectly reading the table in the MySql database. I.e it reads 6 columns, when there are in fact 21. Hence some of the formulas appear to be referring to fields that do not exist, but they physically do in the database. The first screenshot shows AppSheet thinking there are only 6 columns.
The next screenshot shows the physical table definition.
I have raised the issue with the support team and wonder if anyone else has experienced this issue? At present the issue is one application and not any others.