I would presume talking about generated text from SRT etc, where it is putting out a constant luminance but the display is adjusting the whole screen including the constant luminance subtitles based on the RPU in DV-Std.
Would be interesting to known if the same happens for DD-LL, I would guess maybe not, as RPU maybe applied to the video only and then OSD overlayed.
In order for it not to go up and down with the RPU I think would need to interpret the the RPU and apply an inverse adjustment to the luminance of the generated subtitles - maybe something the CE team can look at down the line.