Setup

Set Jenkins to have one node (master), and two executors.
Create a throttle category my-category with global max 1.

Create freestyle project A.
Build step `sleep 10`.

Create freestyle project B.
Throttle on my-category.
Build step `sleep 10`.

Create freestyle project C.
Throttle on my-category.
Build step `sleep 10`.
Post-build, trigger downstream project D.

Create freestyle project D.
Block on upstream jobs.
Build step `sleep 10`.

Trigger A, B, C, and D, in that order.

—

Expected

A starts.
B starts.
A finishes.
B finishes.
C starts.
C finishes.
D starts.
D finishes.

—

Actual

A starts.
B starts.
A finishes.
D starts.
B finishes.
C starts.
D finishes.
C finishes.

D does not wait on C.

—

I also tried lockable resource plugin with the same setup, but it didn't work either.

I do not know of any workaround.