It’s because the person who queued supp had the supp skills of whatever elo your jungle is in. The reason you got a warning is because you switched roles and unbalanced the “balance” of the match where each person in each assigned role had very similar MMR.
You got a warning because you unbalanced the game by switching with a plat lvl jungler who was queued into a gold lvl MMR because of his/her supp MRR. It makes it unfair to the enemy jungler who might be a jungler main and actually has gold MMR in jungle and it gives your team an unneeded advantage because the “gold supp that is actually plat jungle main” has more than usual game knowledge and can still play the assigned role without much difficulty.
EDIT: on the positional match making part, I think it goes back to regular matchmaking before the positional ranks were added.this may sound like a good thing but I think it might even make your chances of getting autofilled higher, which is bad because your total MMR is high enough that you could have silver lvl top lane skills and be in a plat+ game lol.
EDIT 2: the examples I gave with the MMR and supposed plat jungler with gold supp rank was entirely fake