Because figured bass are global items by default, they are attached to the instrument rather than the specific staff. At the moment there’s no workaround for this, other than to use an instrument that has only one staff to begin with.
First of all, can I please say what a pleasure it is to work in Dorico. I have been writing out a mass by Corrette for a Christmas concert (sorry to mention the C word so early). It has been over a year for me, to get to a stage where I actually feel comfortable working with the software. I think a lot of that is down to the work that @Lillie_Harris has done on the manual. Congrats.
But I too noticed that figuring was disappearing when the option to “Allow individual staves of multi-staff instruments to be hidden” is checked in Layout Options. Is that something that is going to be addressed any time soon? Given that not all the Corrette is simply a figured bass, some of it is written out on two staves within the same flow, I will need to be creative to work round the layout.
It’s not something that we are presently working on, Fergus, so I’m afraid you will be dealing with the workarounds for a little while longer. Sorry for the inconvenience caused!