Clipping in HDR space is normal, at least for consumer displays. Dolby Vision movies are mastered at something like 10000 nits, which is something no(?) consumumer display can show. Manufacturers, like LG, have to make some decision in how to handle this deficiency. One option is to 'compress' the brightness range into the ±800 nits that the e.g. the LG C2 can display. Except for the highest of highlists, most content is still mastered around SDR-ish brightness levels of 200 nits. A simple compression of 10.000 nits mastered content into a 800 nits range would therefore result in very dark images.
The solution at least LG has chosen is to clip the highlights. It simply does not matter much if a highlight like the sun is displayed as a single very bright area, as opposed to some spots that are very very bright, next to spots that are incredibly ultra bright. A max brightness sun will suffice.
This is why the concept of 'paper white'.matters in HDR space. You set the level of 'SDR max brightness'. at e.g. 200 nits and ideally the display then shows highlights as bright as possible (within reason for the particular display).
Obviouslsy this does not work when translating SDR content into a HDR format, as your 'SDR max brightness' also contain the highlights that you ideally want to display at HDR levels. The "Auto HDR".settings of Windows, xbox and LG's own TV's rely on (AI) algorithms to properly detect which details from the SDR source should be highlighted. The Mister cannot and should not attempt this, but instead optimise its output for CRT simulation.
The current mister implementation of showing #ffffff as max brightness is dubious. I suspect no one actually wants to be blasted with max brightness for everything that is white on the display. I have not checked how the colour adjustment settings are implemented, but I suspect the current setings mess with the SDR output, before translating that to HDR, destroying a lot of dynamic range in the colours in the process.
In addition, the current settings are not sufficient for TV's from e.g. LG, since they set the max brightness of HDR apparently at 10000. This is not so much a problem for HDR content, but it does mean you cannot simply map SDR content to the entire HDR brightness range, since that results in (severe) clipping.
It will be some time before we get there, but I do think this new addition is a great step forward for Mister.