Many TVs have different settings for the native display and each HDMI port. They can usually be toggled to use the same settings across all inputs. But it's possible the settings for your Roku Stick in the HDMI port are different from the primary display settings. Since you said 6 Series, I think that's the TCL brand. I've never used one of those, so don't know specifically what settings it might have. Even though it's using the Roku OS, each manufacturer can tweak the menus.
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