How would the compression change for OLED vs standard LCD etc? I guess it could be generally bad but only noticeable on a OLED?
I'm not an expert on panel technology, but I think because LCDs have a backlight, the issue isn't as pronounced there. It just kinda washes out.
My best guess is their choice of codec and compression settings predate OLED availability and they don't want to re-encode and just expect people to upgrade to their 4K plan. I've never had their 4K plan, so I can't say whether that truly addresses the problem.