Thanks.This has happened to me also with FS4K.
The resolution reverts back to 1080p on itself.
I think it's due to some handshake issue with hdcp protocol while the FS4K is alredy booted up, and the TV is switched on later.
Then I powered up the FS4K from the TVs usb port which solved the issue.
Now the FS4K boots up only after TV is switched on. The resolution never got changed again!
Is Mi TV able to provide enough power to Firestick 4k (or max)?