Ditto.
Jenkins 2.32.1
I have 10 executors on my Jenkins installation
I set up 3 jobs, which all have these TCB options set:
Throttle this project as part of one or more categories (set)
Maximum Total Concurrent Builds = 1
Maximum Concurrent Builds Per Node = 1
List of parameters to check: Expensive

If I queue up all 3 jobs, they all three run simultaneously. I expected them to block each other.

It is impossible to diagnose the ticket without having information about Job types you are trying to Run. Please provide configuration samples. If these jobs are Pipeline ones, likely they won't work properly, see the recent changelogs

It's been a while, so I don't have the jobs in my Jenkins environment anymore, but the short answer is:
Three new/empty freestyle jobs.
Each has Windows batch command: timeout 10.
Set the Concurrent Builds plugin as described.
That's it.
My jobs were as minimal as possible to ensure that it was not the fault of another plugin

thanks!

Same issue here.

Created a category, maximal total concurrent build to 0 (unlimited), maximum concurrent builds per job to 1.

Four jobs configured to be throtlled as part of one of multiple category, using that category. I still have two of those jobs running concurrently on a node with two executors.

(Added information : those are matrix project jobs, set to throttle on master build, and still having several master jobs running despite the expected throttle from the given category).