My 5700 XT doesn't seem to like my Samsung 4k TV. By turning on GPU scaling in radeon settings (under display tab) helps a little but it still acts strange sometimes with black screens and stuff. I have similar setup to you with my other display a 144hz freesync.
Previously my Nvidia card worked with this TV no issue at all, makes me really regret switching to AMD. When the card works properly it gives nice performance but it's not worth all these stupid driver issues.