Confirm in local preferences, all is set to max, 100 of processors, including allowed memory max use, and disk AND VM. Then add <ncpus>24</ncpus> tag to cc_config.xml's <options> section to force the hand (restart client of course). Also, the OS itself in Task Manager under Performance has to show Cores 12, Logical Processors 24, but the cc_config.xml tag will just make the client run 24 jobs, even if there are less processors, fake it.

1) VM in the context of regular BOINC means VIRTUAL MEMORY, the good old swap file. Mine is set to allow use of 75% of it, and it is 20GB or so ATM.
2) Great excuse, neither am I. The manual to cc_config.xml is here: http://boinc.berkeley.edu/wiki/Client_configuration. When you follow the instruction: "Beginning with version 7.4.26, the BOINC Manager has a dialog to make it easier to edit the logging options." When that is opened and saved, it creates the cc_config.xml file for you, if not already existing, in the BOINC data directory, path printed in event log. Then it's easy to find or add the <ncpus> tag to the <options> section.
3) As said, the tag will force BOINC to run with 24 cores, BUT, why don't you paste the startup section of the event log in a reply, first 50 lines, for then we can read what BOINC knows and doesn't. Then we can all read what is/is not what bOINC believes.

To be clear, the three other systems I have running (shown in the signature) are all running Win10 without this issue, so hopefully a reinstall might just fix it.

So a re-install fixed the issue? I wonder what the issue was in the first place though, could have saved someone in the future the trouble of having to re-install. Especially if they need to do backups, etc of data.

My first thought was that somewho the OP had used an image or a config file from the Ryzen machine in their sig, which kept the 16 thread count.