Wow lot's of information all of a sudden!
Unfortunately I don't remember who traded this Rayquaza to me @ICanSnake, but I can confirm that the proof link you shared for yours is the exact same I have for mine!
I'm still green on legality checking as this is all new to me so I might be wrong, but it's my understanding that just like any other Pokemon in the game, Mystery Gifts and Events roll a random PID (within the specifications of the WC for shiny locked, IV locked etc) and EC, so the fact that your Rayquaza has the exact same values as mine makes it seem like they are in fact the same Pokemon/file..
If my assumptions are incorrect and PID/EC is locked and set though, then could it be that yours is a case of passing off proof of a different Pokemon that was actually redeemed, for the one you were traded, as @theSLAYER mentioned..
The fact that you know the redeemer and obtained the Rayquaza from the person they redeemed it for or makes it more weird.. But since you say that they had a history of trading self hacked events it could very much be that they redeemed a Rayquaza at the event but traded a genned/incorrectly injected (including theSlayer's suggested method above) one and passed the proof off as there's no sure way to tie it to the actual Pokemon unfortunately . Why would they do that though? ¯\_(ツ)_/¯
Nevertheless, I didn't mind the hijack, but the issue I raised is in fact that of HOME changing this Rayquaza's OT when transferred and if this is 'normal' behavior, and if more (and which?) events might be affected by this same issue? If you have access to your Gen 8 saves, could I ask that you transfer a copy of your Rayquaza up to Home and SwSh and check to see if it suffers the same bug ICanSnake? I might just inject one with the WC here and after checking all is ok in pkhex, transfer it up and see if it changes the OT as well. If there are conflicting results there and no change happens, it might mean both our Quaza's are haccs
Also not sure if relevant but here's the pertinent screenshot from my Quaza @theSLAYER
Cheers!