Custom time signature properties not applied to subsequently added instruments.
If I create a custom time signature (say with overridden displayed value or beam defaults), then add a new instrument, the time signature properties are not applied to the new instrument.
Create a new time signature in the Master Palette. Set it to 9/8, and to make things obvious set the displayed value to 99/88. Add it to the score.
Add a new instrument to the score.
Notice that the new instrument has its time signature displayed as 9/8, not 99/88.