Ok so it appears the mystery has been solved!
I tested by sending various pokemon (including the actual untouched Rayquaza) from Bank to HOME. I edited a few shiinotics to be KOR lang tag and gave them an OT with KOR 7 characters (limit is 6) consisting of 7 Korean characters, 6+ a number and 6+ a latin letter, and pkhex promptly flagged the OTs like you showed @theSLAYER !
To my surprise though, none had their OTs changed by HOME upon arriving there
I then went to ask the person I traded the Rayquaza what exactly did he do (if had he traded it via Home specifically), and they told me that they re-checked and the quaza has in fact got its original OT in his game; the pk8 he saw in his pkhex was obtained via dumping through a sysbot, as he has no direct means of save manip in gen 8. This then explains the issue, it's the sysbot who altered the OT, prob because of the exact reason we were discussing here (OT too long for KOR?)!
Cheers!