Confirmed by myself after a user reported this on WP:VP/T. It's not currently possible to disable the Syntax highlighting form the Beta panel in your preferences
This might be related to having used the button in the toolbar to enable/disable the syntax highlighting, or to having used the "Yes i'll try out "-dialog, and then having inconsistencies between true, 1, "1" etc in the preferences.
It's now working for me. As far as I'm concerned, you can close it. (TheDJ raised it on my behalf).
Hmm, maybe a master vs. slave database race condition in the logic for "automatically enable all new beta features" ?
Is it only for Syntax highlighting? If not, please mark this as a duplicate of T170055: Impossible to turn off certain BetaFeatures when "Automatically enable all new beta features" is selected.