So, I filled in a part of the moat and extended the barn over it. While I was doing that I came across a random boar which I lured into the barn, over the filled moat. That one tamed just fine.
I then set the other two *bugged* boars free, and lured them in the barn again, over the ground, and now they're taming properly.

So the issue seems to be moat/ground related after all.

I'll make sure to put one pair safely under lock and key, too.