I have an 8 "real" core AMD Ryzen 3700, which is running quite well, but....
Periodically BOINC will see all 8-cores, then a few minutes later drop this to 6-cores.
This confuses me, is there anything I can do to ensure all 8 "real" core are available to BOINC?

I've got "use at most 50% of CPUs available (I assume this means 50% of the cores?) set, along with "use 100% of CPU time"

Is it depending on certain tasks that you run? Which tasks does BOINC run when it drops to 6 cores?

There can also be separate settings when computer is in use or not in use.

I've got "use at most 50% of CPUs available (I assume this means 50% of the cores?) set, along with "use 100% of CPU time"

50% of CPU's means of all cores, not just the real ones. I find maximum throughput at 50% -1 cores in use so 15 out of 16 real cores.

Answers to questions:

Is it depending on certain tasks that you run? Which tasks does BOINC run when it drops to 6 cores?

Independent of tasks being run, or number of tasks (provided one is running.

There can also be separate settings when computer is in use or not in use.

I run an "all or nothing" set of settings - if the computer is being used to reach my threshold all BOINC processing is stopped. The dropping of cores, and their reappearance can happen when the only BOINC is active, with one or more tasks running.

50% of CPU's means of all cores, not just the real ones. I find maximum throughput at 50% -1 cores in use so 15 out of 16 real cores.

Thanks - I did try with letting BOINC use 7 of 8, but it still did the random dropping to 6 cores, followed by returning to 7 so I left it at 8 of 8 (not 7 of 9......)

Now even more confused,bust sort of relieved.
After a few overnight shut down & restarts BOINC appears to be behaving itself and sticking to 8/8 core.
I could say rude words about computers......