Establish the actual room temperature
Use a separate room thermometer in the occupied part of the bedroom, away from the discharge air, direct sun, and another heat source. Record the temperature, the selected setting, time, outdoor conditions, and what the other heads are doing. Keep the door in its usual position. Note whether the head appears to stop heating while the room continues warming.
Ask the technician which sensor controls that head and whether an installed remote sensor or controller changes the sensing location. Have them compare the temperature the equipment sees with the room measurement. Do not enter installer menus, move sensors, or disconnect anything to test a theory.
Review the small room and shared system together
We would want the technician or designer to check the bedroom's room-by-room heat calculation, the exact head size, the outdoor unit's minimum output, and the approved connected-head combination. Ask how the system behaves when this small room needs little heat and other rooms still need it. Have them review relevant control settings and whether an installation or operating fault is contributing.
Request the actual model guidance and measurements behind the explanation. The total advertised capacity and the number of heads do not answer whether the combination suits this bedroom during milder weather.
Our preferred correction is the least disruptive one supported by findings. A sensing or setup correction deserves consideration before replacement. If documented room needs and equipment behavior show a design mismatch, ask for options that address that mismatch, with a clear explanation of what each option changes. Simply fitting a larger bedroom head should not be accepted as an unexplained fix for overheating.
Your next step: Send the temperature log and all model numbers to a qualified ductless technician or designer. Request a written explanation that connects the room, controls, and shared system to the proposed correction.