An admission: I don't know that it's a bug. It may be a deliberate change in 11 that breaks the ability of Calibrite Profiler and DisplayCal/Argyll CMS to properly detect the monitor's characteristics. Calibrite hasn't caught up with 24H2 yet, and Argyll is open source.
Eureka!
It's not a bug, it's a feature.
There's a system setting (system > display > color management) "Automatically manage color for apps". It was turned on.
When I turned it off, DisplayCalArgyll CMS functioned as before. 100% coverage sRGB, 99.99% Adobe RGB, 96.75% DCI-P3.
It's not what I'd call intuitively obvious. I got a hint from a fairly unclearly written (IMHO) link in another forum.
Windows 11 24H2 Update: AMD Users Face Color Profile Challenges | Windows Forum
I'll have to check whether the issue has anything to do with AMD CPUs. (I have one on this system. Other PC, Intel Gen13.)