After updating a webmap from ArcGIS Pro (3.1.0), layer fields are becoming unvisible, even I didn't make any changes in the visibility settings or any changes in the WebMap.
In your post you say you did not make any changes to the webmap then what updates did you make to the web map?
Because I saw the problem with updating webmap and visibility of the fields and I wasn't sure, what was the reason for that, I've tried (just for testing purpose) to update/save webmap without any changes. And had still the same problem.
Does the layer have any temporary joins?
No
are the layer fields visible in ArcGIS Online?