2024.1.4 is the latest release at time of writing so you should now be able to switch frame rate testing off in the Experiment Settings and manually specify a frame rate - at the bottom of the Screen tab:
Keep in mind that the “Frame rate” control is hidden when “Measure frame rate?” is ticked as the frame rate specified there wouldn’t be used (PsychoPy will use the measured frame rate) - that may be why it seemed to disappear?