erratic master fader

what have I done?

Master fader sometimes behavies erratic - meaning I fade down for blackout - and on fade up - master level (in red text top of screen) gets stuck at some seemingly arbitrary value - I fade down again and all the way up - and gets stuck at a different value - fader itself is read fine 0-255 in diagonstics... all other faders work OK - this only happens sometimes - not sure how I get to this state...

only been on Ion for a few months - so stumbling into something I may have done to myself?  not set any long fade times anywhere... cue list is out.   ideas on what this is?  (happened from 1.7 through 1.8.1 beta)

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