Accent colors ruin HiDPI text globally

This is a bug I got to observe since the introduction of accent colors but always thought it’s such a striking problem, someone will probably fix it soon. But it is still present in Plasma 5.27.8 for me.

When I set to use an accent color, all my programs are falling back to non-HiDPI font rendering. Both Qt and GTK. Relogin doesn’t help. Makes no difference if I chose the accent color from my wallpaper or a custom color. Only setting the default non-accent followed by a relogin does reset my HiDPI fonts.
X•org session on 4K resolution with global HiDPI slider at 162,5%, this sets the fonts DPI to 162.

I even made a demo screencast:
(Downscaled to FHD)

KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.11

Original bug report: 470138 – Accent colors ruin HiDPI text globally
Nate told me to try it here. “Perhaps some ancient stale config left over from years ago.”

Interestingly, I’ve found it also on another PC in our house with Plasma 5, originally upgraded from Plasma 4, like on mine. Same HiDPI corruption as soon as I chose to use accent colors.
Tried it in a new user profile on my PC, it hasn’t happened there. So this must be some old config segment KDE devs don’t see anymore because apparently no one but me is using the same config profile for over a decade.
I don’t want to try it in a Wayland session with my main user, because the last time I tried the Wayland session two years ago with my main user, it has altered some other core desktop settings which was a pain to set back.

Does someone here have an idea what could cause this HiDPI breaking when enabling accent colors?