Block on upstream projects does not work

Do a fresh install of Jenkins. Make 3+ executors.

Create project A, with downstream B.
Have it `sleep 15`.

Create project B, with downstream C.
Set it to block on building upstream projects.
Have it `sleep 30`.

Create project C.
Set it to block on building upstream projects.
Have it `sleep 30`.

Trigger A, B, and C together, in succession.

As A builds, notice that C is blocked, and waits for the (transitive) upstream project A to finish.

However, once A finished, B and C both start!

Only B should have started. C should have stayed blocked until B finished. Instead, C behaved as if A – and A alone – were the upstream project.