I encountered this problem on the first day of the public (not beta) release of Sequoia 15.0. Immediately contacted support and have talked to several advisors since with no resolution in site. Editing to add that the 4th person I spoke to elevated the issue to an engineer about a week ago, but the engineer hasn't gotten back to the advisor yet. This happens on my M1 desktop all the time and my Mac Laptop when it's hooked up to an external monitor. It doesn't happen on the laptop's internal screen.
I'm working with an LG C3 color calibrated monitor that's hooked up directly to the HDMI port (not going through an UltraStudio type device), but I tested it with 3 consumer-grade HDR monitors ranging in price from $300 to $1500 on both machines.
When working with an HLG project, all HDR clips appear dark in the browser and the viewer when I hover over them in the browser and the timeline. When I switch the project to PQ (including setting the proper monitor's metadata in the project settings), the clips display fine in the viewer if I hover over them in the timeline, but they display dark in the viewer and the browser when I hover over them in the browser.
I tested several different projects using footage from several different cameras, and they all have issues. If I open those same projects on a Mac with Sonoma, they work correctly.
I have been working with HDR timelines for over 5 years and know how to probably set up the project types and edit/conform footage between color spaces.
To test more, I went to an Apple Store and tried to duplicate the issue on a Mac mini and a Mac Studio that were hooked up to the $1599 Mac Studio monitor via USB-C, and I couldn't duplicate it. So, it either seems to be an issue with the computers talking to the monitor.
I typically have edited and exported my HDR projects in HLG, but I have since switched to editing in PQ. The clips are still dark when I hover over them in the browser, so it's difficult to mark in/out points, but they look fine once added to the timeline, so I can work with it.
This issue does not occur with DaVinci Resolve Studio 19.0.1 or 19.0.2, which was released yesterday.