Screen temporarily brigthens when displaying PGS / graphics based subtitles

I’ve noticed this during dark scenes in films with subtitles.
The entire screen noticeably brightens when the subtitle is visible, and then dims when it disappears.
It’s not immediate, the change in brightness is transitional, about a quarter of a second long.

I have all the adaptive brightness settings, and any settings of that nature turned off on my TV.
Besides, considering almost all subtitles are white, an adaptive light would surely dim the screen when they appeared.

I’m using the latest build (as it was) from around March this year.

Any ideas what’s causing this and how can I fix it?

I have seen this on an UHD LG with adaptable back-LED brightness. Apart from disabling Adaptive brightness completely I had to use dark grey subtitle font color to get rid of this annoying effect. Don’t know if that can be used for graphic fonts, or they always stay in default color…