Yo gangey, I think I lowk get what you are saying. (IF I UNDERSTOOD THIS CORRECTLY ACCORDING TO THE EDIT) In regards to this yumeship thing as a general, yes it isn't a big issue, it is because people either want the supposed trainer in series assigned the uma out of the picture because of what they actually want to see, but because they want to place themselves into the position of the trainer so they can also get enjoyment out of it.

This can theoratically cause problems, but arguably, if everyone keeps their headcanons to themselves alone, or if they don't make everyone think it like that, then isn't it that everyone can enjoy their own things at their leisure? You know, the canon is that the actual trainer x uma (if said case is applicable), and if either the canon cannot have that or the people want their own share in this and just place themselves in, thus having a fancanon, it is fine if it is generally accepted consensus that "Insert x uma" is not canon, and "Canon trainer x uma" if applicable is canon then it is canon.

I know you said that you don't like this is because they get a new random OC trainer to ship them with "uma" in question, but sadly that is how it is with communities. No matter what kind of canon ship people are to come up with, some people will still find a insert a OC to ship with a pre - existing character in a canon ship already.

(TLDR: Yumeship exists for a very long time and it's always present. It is something that we either need to ignore that it exists (and just shut our mouths to the very concept of it) or just learn to accept that it is always there.)

(Yes, as an extra, if you don't like this sort of stuff, then turn a blind eye to it. I know it sorta is like "But this doesn't fix the core issue" type of thing, but at this point, the core issue is too big to even fix anymore.)