Since I had some time today, I planned to level up so much that I would insta-kill the snail to workaround the problem ...
(exp via gauntlet, easiest way is to go in room with ranged attack monster, and kill them with poison while always moving)
It worked, and weirdly now I am way higher level than the snail, it doesn't seems to bug ... I let them attack and it wouldn't freeze.
So still don't know what the bug is, but it appears to happen only when you are under level. (at level50 it was working fine, bug on level33 I think -- I might try to find out the break point on next remort)