How do weapon rank gains on promotion work?
You may have noticed that, when setting the base weapon ranks of characters and classes, you set them with numbers on a scale from 1 to 255, with 1=E, 31=D, 71=C, 121=B, 181=A, and 251=S. These are actually base values for weapon experience, or WEXP, with those numbers I just listed being the default cutoffs for how much WEXP you need to reach each rank.
WEXP gains on promotion are determined by the difference between your promoted class’s WEXP bases and your unpromoted class’s WEXP bases. If the unpromoted class doesn’t have access to a weapon type the promoted class has, then this just straightforwardly gives you the base value of the promoted class, as the unpromoted class’s base is 0. But for weapon types you already have, it takes the difference in class bases – if you promote from a class with base 31=D swords into a class with base 121=B swords, you gain 90 sword WEXP.
This unit is suddenly skipping to S rank in a weapon type on promotion, and I know I didn’t set the promotion weapon rank gains that high.
This is an issue that can occur when a unit has less WEXP than their unpromoted class base. Since player units can have personal WEXP bases that override class WEXP bases, it is possible for a unit to, say, start with E rank where their class’s base rank is D. If they still have E rank when they go to promote, this causes errors, presumably because it tries to subtract the class base from their current WEXP value and underflows. Make sure your units always have at least as much base WEXP as their class base – or rather, that your class WEXP bases are always lower than the WEXP bases of your units (aside from being nonzero, class WEXP bases only really matter for promotion purposes, after all – generic autoleveled enemies will automatically gain enough WEXP to use the weapons they load in with so long as those weapon types can be used by their class).