Excel formulas ignore cell value until edited, even if not changed

In an Excel document with two worksheets, I have multiple columns with formulae like:

=IF(ISBLANK($E105),"—",XLOOKUP($E105,MAPPED!$A:$A,MAPPED!C:C))

The formula returns an error although there are values in MAPPED.

Value Not Available Error
A value is not available to the formula or function.

If the keyword cell is edited, without even changing anything, the formulas "activate" and return the expected result. I never seen this behavior. What is going on here?!

This is on a Mac with latest Excel 365, both with the native app and in the browser.