"Why does this happen? "

Oversize riser pipe?

The flow restrictors also affect the drain time, but it sounds like you have picked a shower head that siphons the entire riser.

Usually only the water from the peak of the riser (and/or shower arm) to the head itself drains out.

Do you mean that all the water from the shutoff until the showerhead is being sucked up and out? Also, according to this the showers with longer shower arms would take longer-is that right?
It sounds like you think the problem is the choice of showerhead, not the plumbing.
Thanks,
AY

AT a minimum the water above the shower head needs to drain.

If you have a long shower arm it might take a few seconds, but 20 seconds is approaching excessive.

The whole shower installation plays in drain time, shower riser can cause issues, the length and size of the shower arm, and the shower head.

One way to isolate what is causing the problem is to remove the shower head, then briefly turn on the water.

Note how long it runs out after the valve is closed.