After running a 5x multi-battle on stage 12-6 brutal (with xp boost) i got this strange result in the summary:

XP Gained:

Elhain: Max Level
Treefeller: 65k
Eviscerator: 62k
Raider: 56k
Total: 183k

XP gained per champ should be 8800*5 = 44k.
Total should be 8800*5*3 = 132k (xp gained, not counting the farmer).

None except the farmer were at max level, the rest being around level 20, so none of them stopped gaining xp during the run.

Second try, again 5x multi-battle, same stage:

Elhain: Max Level
Treefeller: 52k
Eviscerator: 50k
Raider: 52k
Total: 154k

So I took three fresh Incubi and ran them (with the farmer) 5 times through 12-6 brutal. Each of the five runs they each gained 8800xp. At the end all three were level 15 with 4344xp into the next level.

Then did another 5x multi-battle run (12-6 brutal, xp boost) with 3 new Incubi. Afterwards these were level 15 with 4344xp as well so it seems the champs are getting the xp they should.

However:

Elhain: Max Level
Incubus: 89k
Incubus: 89k
Incubus: 89k
Total: 266k

The xp summary needs another look.