My 43" TCL gives me a overheating warning but only when I am viewing videos on YouTube! And then again only on certain channel subscriptions on YouTube. If I switch to another video the warning disappears. This only happens when I am watching YouTube and never on any other channels or streaming service.
This would appear to be a hardware related issue. Have you tried contacting TCL support yet to inquire further? I'd suggest starting there to see what options or suggestions they can provide. Each Roku TV manufacturer provides direct support for their products running the Roku OS.