I am probably missing some information to fully understand...
Someone on the XPZ Matrix server has reported that the manual promotion feature had a knock on effect that probably wasn't intended
That's possible.
I fixed about a dozen bugs in that PR before merging, but there may be more.
unpromotable soldier types (like XPZ repentia and pirate dogs) are now counted for the purpose of determining additional promotions for other units.
How did it work before this feature was added?
This has a significant gameplay effect in XPZ, since it substantially increases the effective costs for unpromotable troops that are balanced around being low cost.
I don't understand this.
How does any of this increase the cost of anything?
I need some example to understand.
Since it seems like an accidental consequence of a feature addition, it seemed right to treat it as a bug.
Yup, thanks for the report.