You've got me confused now, lol.
Here's how my core assignments are currently set up...
cpu 0 / 16 UnRaid
cpu 1 / 17 UnRaid
cpu 2 / 18 pfSense
cpu 3 / 19 pfSense
cpu 4 / 20 Legolas
cpu 5 / 21 Legolas
cpu 6 / 22 Legolas
cpu 7 / 23 Legolas
cpu 8 / 24 Dockers
cpu 9 / 25 Dockers
cpu 10 / 26 Dockers
cpu 11 / 27 Dockers
cpu 12 / 28 Plex
cpu 13 / 29 Plex
cpu 14 / 30 Plex
cpu 15 / 31 Plex
So, they way I understood ISOLCPU works was that I could stop UnRaid from using cores that I wanted to use for something else, i.e. VMs and Dockers. So I added this to my config...
isolcpus=2-15,18-31
The behaviour that I'm seeing is that with the command in place, my Plex server only uses 1 core (Core 12). Without the command it uses all 8.