I had the same. This seems to be a bug in SB. If you add a complex expression to a hard button press, SB sometimes removes it on rebuild because it thinks some signals are missing. Sometimes assigning an automatic new join number to the button will fix this.

You can try to put the expression into a global preset and call the global preset from the button press. In that case at least its easier to restore the programming if it's gone.

Another way, using the "Persist hard button programming from this page" and on other pages "use the persistent buttons if a previous page enabled them" function on pages makes it easier to program because you only have to set the button programming once and keep the programming across page flips.

If you have a lot of rooms and the volume logic becomes complicated (and you can program SIMPL), its probably far more stable to write a SIMPL logic module for volume control and import it into SB. In this way you can also make a feedback signal (e.g. volume bar per room) if the amp supports feedback on volume.