Do you get a log message similar to “Contrapasso - video.level limitation applies: 120 > 42”, or what else does the log show?
I’m not at home and can’t check my own chromecast.xml file, but one I found on the net has this section:
<VideoCodec name="*">
<Limitations>
<UpperBound name="video.width" value="1920" />
<UpperBound name="video.height" value="1080" />
<UpperBound name="video.bitDepth" value="8" isRequired="false" />
<UpperBound name="video.level" value="42" isRequired="false" />
</Limitations>
</VideoCodec>
If you have this as well, maybe you need to add a section setting a higher video.level specifically for HEVC. Or maybe that already exists in the current Chromecast XML file.
Disclaimer: I have no HEVC-capable devices so I’m just speculating here…