Hi,
thank you for reaching out about this issue and for the clear repro steps. I found that this texture encoding change was introduced sometime between 5.3.2 and 5.4.4. I could make the mask textures match fairly closely (there are some compression artifacts noticeable on magnification) between the different engine versions by changing Encoding Override (under Texture > Advanced > Source Color Settings) to Linear on 5.3.2 and to sRGB on 5.4.4.
I’m still investigating if this is a bug or an intentional change in the engine, will post an update soon.
Thanks,
Sam