R only ever uses one CPU (even in parallel) on RHEL 7

Awhile ago, I installed R 4.0.2 on RHEL 7 (8-core machine) using the instructions here. I have since noticed that, no matter how many R processes I open, they only ever get executed by the first processor on my machine. (More details on my StackOverflow question here.)

The behavior I'm seeing matches almost exactly this StackOverflow question from 8 years ago.

Just today, I installed R 4.0.3 using the same instructions and am seeing the same behavior on that version.

Does anyone know what might be going wrong with my build? Any help would be appreciated.