I have the same problem. I allow them to be loose. I guess if we used them enough, there'd be less of a problem. The older set of iyoroi chisels I have (I guess it's actually my newer set, but the old set of ouchi chisels i have didn't get their hoops set until last year), I set them in the summer and had to trim the handles and of course that resulted in all of them being loose.

I don't know if there's merit to soaking them with water and then trying to replace the water with non drying oil, but I think I'm going to do that.

I had a gumi handle actually release a piece (even though the hoop is set properly) of fairly large material the other day while pounding out HBDT , but the chunk came out at the top of the handle in the hoop.