I just got blinded twice in the pitch black darkness of my bedroom because Memmy randomly decided it's time for 2 seconds of light theme.
Seems to be triggered by switching between Apps.
Turning off the automatic light/dark switching fixed this for me. Having automatic switching turned on in the latest beta causes light mode for a few seconds when returning to the app in system dark mode.
Not only, sometimes the bottom menu flashes in light mode, sometimes just the top panel, sometimes the whole app. I will go now with the dark theme all the time to be safe.