I am using Bullseye with old Rpi2
brightness adjustment is a slider in "edit view "menu
it does change brightness of the font in the display, it just does not save it

I gave up on MMM-2Day-NOAA-Forecast and switched to another module - MMM-3Day-Forecast.
Seems to work fine for now

I’ve resolved it. Google wanted my credit card number :) for API.
It works now

@KristjanESPERANTO Yes, I have

@mishman
MMM-MktIndex looks like was updated 4 days ago.
I have reinstalled it and now everything works. Thanks to the developer!