The bathroom rad is through the wall from the hot water tank in the bedroom, so IMO the bathroom rad should be hot when water is hot. Bathroom rad has thermostatic inlet valve. (The "always-on" rad is downstairs in the front hall.)
There are no cold spots in the rad. Entire system was power-flushed recently, but that did not cure the on-off-on problem I would be grateful if someone would suggest a solution.
A few years ago, the bathroom rad was "alway on". But when our Vaillant boiler was installed, the "always-on" rad was changed from the bathroom down to the hall.