Are you use the MAME from mamedev.org or using the MAMEUI version with the Windows Frond-End?
Have you tried a fresh install and made absolutely sure you have no other saved ini or cfg files which may be later on the read chain for the machine and be loading a previosuly saved/default setting?
Multi Config information:
https://docs.mamedev.org/advanced/multiconfig.htmlThat said, testing 0.220 just released:
- Editing mame.ini to have "pause_brightness 0.2" then running MAME with a game (mappy) shows me a properly dimmed screen
- Issuing command line "mame64 mappy -pause_brightness 0.2" accomplishes, again, a very dim pause screen when used.
So, please, check all your config files or try a fresh install and see if this happens still.