Names are probably the most annoying part of a mod (in my opinion).
They are difficult to control, because they are coded in several places.

I guess that your problem comes from the fact that the general's name is not considered as a name for a king. In the HEROES file each faction has a certain number of names allowed for kings, from the list of forenames below.
In addition, the number of precedent kings is encoded in the startpos file.

Unfortunately I cannot give you a solution (only, take the loyalist faction ). In fact, I am not sure if the same problem would appear in a non-modded campaign.

Good luck in any case.