In 8K and the most demanding 4K modes, the EOS R5 stops recording once internal temperature reaches its threshold, then requires time to recover before the full recording duration is available again. The camera has no fan; heat is managed passively.
Firmware 1.1.0, released on 27 August 2020, states that temperature detection and shooting time control in video shooting have been improved, and that total shooting time is improved when short recordings and power-on/off cycles are repeated at room temperature. It also added messaging when overheat control is on alongside an external HDMI monitor, and corrected a case where the available recording time was displayed incorrectly when date, time and zone were not set.
The status here is acknowledged rather than resolved, because the firmware changed how the limit is estimated and controlled rather than eliminating it. Any ownership report about this camera overheating is only meaningful alongside the firmware version it was running, which is why firmware is recorded on every report.