Looking at this Patchouli Type yukkuri I feel that my theory that even yukkuri of a same type can be of different species. This is the effect of having so many different artist with no different species solid reference. What this means of course is that one yukkuri will not always act like another yukkuri, so they can never be out of character.
"same type, different species" isn't what you mean by that, I think.
Even though a yukkuri species has a general set of traits that tend to be expressed, there are differences between individuals of the species. Thus, you have "greedy shithead Marisa" and "hard-working fatherly Marisa", etc.
Best not to ascribe it to a failing of the fandom when a much simpler explanation will do. Occam's Razor.