Actually, I seemed to have something like that... On Burna Boy (the Incinerate build)... Whenever I shot at a creep, it seemed to do no damage until melee range. Then, when they get into melee range, they just go BOOM!.
Except, that's only in mid-late game creeps. Early game creeps seem to take damage fine.
Its the same for me, but it doesnt matter what level of the creeps. And it seems like its random, cos sometimes it hits sometimes it doesnt. Very annoying.
Btw, great map. I host it like 2 times a day, but thers not a huge rush of people joining, maybe time for a new version?