When controlling with a UT55A/UT52A/UT75A with the PV Hold function, if the input is disconnected while the PV value is being held, is it supposed to display as Burnout (b.out)?

Yes, that is working as designed.
That is displayed so that you can see that a burnout occurred even while holding the PV value (HOLD).
In addition, Retransmission Output the held PV value while burnout (b.out) is displayed.