I went ahead and saved that playthrough so it wouldn’t be replaced by another auto-save and started a new campaign. The same thing happened every time. Restarting the game fixed it but the playthrough I saved still has the reward bug.
For a “reserve” of mercenaries, I think it’d be nice to be able to leave some of your men in a town and then be able to find them at the tavern if you want to rehire.