That's odd. Try this - on the Labeling tab far right in the Map Group, click the dropdown for More and uncheck "Use Maplex Label Engine" - a window will pop-up saying you will lose any Maplex formatting. Click OK. You will now be using the Standard Label engine. Labels should draw as expected without rotation. If this works, great. But if you want to use Maplex, click the dropdown again for More and check "Use Maplex Label Engine" Does the rotation appear again? Please advise.
Interesting about the letter J. What language is ArcGIS Pro using? English or another localization? Try this - add a new field to your feature class and enter a few edits (text) - one with J and see if the behavior is repeatable. If it is, hmmm. If it's not, then I wonder if the original field starting with a J has a leading value of some sort - a blank, a reserved character, etc.