It's pretty common to need an adapter to strip out the CEC line; the Mister causes many devices to malfunction if that pin is connected. That's how you can have one device in an HDMI chain turn the others off and on, and perhaps change their volume. Whatever the Mister is doing with that pin, an awful lot of devices don't like it.
If you've got a Mister Multisystem, they have a DIP switch to disconnect that built in. Otherwise, you can buy something like this:
https://www.amazon.com/BlueRigger-HDMI- ... 07BFL8TM8/
I don't know if that specific one is any good, or if it's even a good price, I'm just trying to show you what you need. There should be lots of options.
This is another one with an on/off switch for CEC, and which may support higher bandwidth, but it's showing as $20 for me, instead of the $10 for the first option:
https://www.amazon.com/Blocker-Intercep ... 0BC83BD6S/
Another option, of course, is to just leave your MiSTer on all the time. It typically pulls less than 10W.