There are 6 thermostats (Honeywell T8400B1018) throughout the house.
3 are in bedrooms we don't normally use.  I set those thermostats to
55 degrees and close the bedroom doors.  In one bedroom, the
Temperature drops to 55 -- good.

But the temperature in the other 2 rooms stays around 65-70 degrees,
And the floor is obviously warm.  So it seems like heat is still
Circulating.  I tried swapping thermostats between the 55 degree room
And one of the 70 degree rooms, but no change.  I tried turning off
The thermostats in the 70 degree rooms, but no change.  So I don't
Think the thermostats are the problem.

Any thoughts on what would cause this behavior?

The termostat controls a valve that allow warm water to circulate. Sounds like your valve is not closing. Do you have access to where the valves are mounted?

Yes, I do have access to the valves.

While I was checking that, I noticed something potentially interesting. There seems to be 1 "actuator" shared by 2 bedrooms, even though each bedroom has a separate thermostat. Does that mean that if either bedroom calls for heat, then both bedrooms will get it?

Thanks for your reply!

I have never seen one connected like that and think it should not be so. If either room calls for heat then, yes, both would get heat. Each stat should be connected to but one actuator, no need for the quotes, you got the name correct.