FYI not really relevant to reporting the problem:
The idea is a Fighter who kills all the big bads and the other 3 buffing him with Elemental Weapon, Magic Weapon and either Stoneskin or Prot Evil.
Part 2 is that the group doesn't Long Rest and the Fighter can use powerful Elixirs for as long as possible, preferably Bloodlust or Cloud Giant Strength, maybe Universal resistance.

This particular bug is so bad for this particular build that I'm likely going to just start over from scratch. Of course I can respec everyone but I'm not sure that's what I'm looking for. Also you can't respec Race or Background.
As I said, when this happened in another game I just ignored it. In that game I was using Sorcerer Extended Haste for 20 rounds, or Twinned. But that was Tactician Mode and a Hasted Fighter is just ridiculous. So I figured I'd try this in Honour mode, while ignoring Haste except in emergencies.

Anyway, that's just some background on why I am bothering to look into this. Those spells were build-defining. (And no cheese from the camp! Just party member buffs.)