Most likely you are hitting this issue: HDMI screen turns off when sustained high CPU usage on all cores · Issue #1598 · raspberrypi/firmware · GitHub
Due to internal clocking changes the RPi4 started clocking down the HDMI block too much when it throttles (eg when it got too hot or in undervoltage conditions) which led to "no signal".
LE 10.0.1 will contain a fix for that, but you still need to resolve the root cause (too weak power supply).
LE 9.2 was able to visually notify you about undervoltage conditions (by showing a lightning bolt on the screen) but unfortunately that's no longer that case with the new graphics driver in LE10 so you/we need to check the logs (we added the throttled state to pastekodi logs right at the beginning though so we can easily see if there was a power supply issue - these are still quite common).
so long,
Hias