It looks like there was a small regression on HP Dev One with our 6.0.2 kernel, and it is still present with the 6.0.3 kernel that's currently in master staging. This regression causes only one of two daisy chained monitors to turn on and work, even though display settings shows both displays, and I can move windows to the turned-off display. Before I log in, if I unplug and re-plug the USB-C cable while I'm on the GDM3 screen, that brings the gray GDM background up on both of the daisy chained monitors, and logging in after that gives me a session with two monitors working just fine.

I also tested some Ubuntu mainline kernel versions (6.0.1, 6.0.3, 6.0.4, and 6.0.6), and they seem to work as expected. That's what is leading me to believe that this is mostly related to the kernel, even though GDM3 and Xorg are definitely playing their own small parts.

Also, Wayland seems a lot less likely to have this problem. If I choose either the COSMIC DE session or the Pop on Wayland session, both external monitors usually turn on and work without issue.

One last note: when I disable MST on the upstream DP monitor, so that the downstream daisy chained display can only display a duplicate of the upstream display, it seems to work every time without issue. Just wanted to include that finding, in case it points to any more useful clues.