Altium Annotate Schematic Problem
You can lock the sub-part designation if you look in the properties of the part in schematic. 

Select the part, and press F11 to bring up the Properties panel.

Click on the 'padlock' symbol to change the symbol from 'unlocked' to 'locked'.
This will prevent the annotator from changing the assignment.

You can also select multiple parts, and do the same thing:


Alternatively, if you invoke: Tools -> Annotation -> Annotate Schematics, then in the Annotate window, you can check the 'lock' symbols next to each sub-part.


Altium Designer 20.0.9