The issue is terrible and arbitrary behavior. ROBLOX changed joints (they weren’t always like this) so that Part0 and Part1 didn’t matter. Part0 is supposed to be the root part and Part1 is supposed to be the sub part, but with this horrible behavior, the larger part is the root part and the smaller part is the sub part, even if the smaller part is Part0. When both parts are the same size, you run into the issue the behavior being arbitrary which causes issues like this.
You can read more on this stupid behavior here: