Not sure if this is the right place, but here goes.
Judging from this post I found, it seems like 4:2:0 subsampling mode is not supported as an output for resolutions lower than 2160p59.94 (or actually for framerates lower than 59.94 at any resolution, because I didn’t get the signal in my setup with 1080p24 at 4:2:0 either).
If that is really the case, then maybe non working combinations of resolution/framerate/subsampling should somehow be prevented in the settings menu? Because the change in subsampling is not applied until we also change the resolution, but if no signal is received by the tv - only the resolution is reverted, leaving the 4:2:0 subsampling unchanged, which also results in no signal.
But I’m not sure if such changes in settings behavior can be done by CoreELEC, or that is completely up to Kodi.