I have done some more testing, so here the results:
- This issue affects 10th Generation (Comet Lake) and earlier CPUs. 11th generation CPUs (Ice Lake, Rocket Lake) support an alternate way to read BCLK and that seems to work properly.
- The culprit is the Core Isolation feature - when enabled (which is the default setting in Windows 11), it will block a certain interface that needs to be used on 10th generation and earlier CPUs.
So my advice is to disable the "Core Isolation" feature if you you're affected by this and need to see correct BCLK