I’ve seen behavior like this in other windows apps if the display is made more readable by increasing the DPI (or whatever other method might be used) to make a small screen easier to read.
This is almost certainly a side effect of using a binding that sets the value outside the range displayable by the slider. What do you have selected in Options -> Keyboard and Controls -> Control Curves?
You can actually define your own control curve → see the guides section on Control Curves
When the curves are all the same, the slider shouldn’t disappear, but sometimes it is useful to have a different MIDI control curve (e.g. I prefer to have 0 dB at MIDI value 64)
This one just bit me … was scratching my head for a while when the “ball” on my volume slider evaporated.
I am wondering if the default values for Tools => Options => Keyboard & Controls => Control Curves should all be the same? It would avoid this issue. (But maybe the MIDI Control Curve setting is set to “Cantabile (Classic MIDI)” for a good reason? So as not to break legacy setups??).