Pausing X-Plane via DREF works but resume does not (using sim/time/sim_speed)

Question

I can successfully pause X-Plane by sending a 0 to the sim/time/sim_speed DREF, however, I cannot resume X-Plane sending a 1 to the same DREF.  Is there some other magic involved?

Comments

I should also mention that I am sending the DREF data via UDP on port 49000.

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I found

sim/operation/pause_toggle

in the command list.  Tried sending it as a CMND to pause / un-pause and that does not work either.