I can't quite wrap my head around this one. I have a project that was created with ArcGIS Pro 2.9. I have upgraded it to 3.5. I have a polygon layer ('Sub Grid') that is set to show labels using the value in a field, 'GRIDPIN'. That's it. Although the labels are showing up exactly as expected in the map, I am getting a drawing alert.
You can see the field 'GRIDPIN' listed in the Fields section of the Class tab. It definitely exists.

There is no SQL expression set on the label class. There is no definition query on the layer.

Several other layers in this project have labeling enabled, and some of those required more complicated Python logic. I'm pretty sure I must be overlooking something that should be obvious, but I'm stumped.