Can't recruit Maus?

The got the, "nubert didn't come with unless everyone came along" dialogue, but all recruits are accounted for.

Or so I thought. I searched up a list and I was missing Maus. So loaded a previous save and went to the location closest to where a Maus spawns. I captured and spared it, but it didn't show a "recruited" and it doesn't show on the list either.

Why can't I recruit Maus?

I didn't capture them before bc I was horrible with timing the cage drop, so I just attacked them.