I've known about this since LabVIEW 2020 32-bit, but just getting around to reporting it. I observe the same behavior in LabVIEW 2021.
Any code within a timed structure should execute in a single thread. That means the parallelism of a parallel For loop within a timed structure should not work because it is forced into a single thread. That's generally true, except when I put a Wait node within the parallel For loop. The Wait node enables parallelism. That shouldn't be possible within a timed structure.
I attached the example saved for LabVIEW 2013. As I said, I have only confirmed this behavior in 2020 and 2021.