I am building a Python Toolbox out of ArcPro 2.5 and setting a MapFrame_Element Camera scale as well, and originally had the Layout Map Frame element with a Display Options Constraint of Fixed Scale. If this was set to Fixed, the python code could not change the scale, but if it was set to None, and I saved the Project (important if you're doing arcpy.mp.ArcGISProject('CURRENT'), it would change the map frame scale from the map frame.
The problem is that there is no indication of this fact. It is very implicit (maybe it should seem obvious) Thank you @ThomasKölbel for making this point evident.