That’s a TWRP design decision. You might consider to use a tipatched TWRP version. But anyway - I think TWRP is unable to decrypt (and so backup) the internal storage of the “s” version (see e.g. also here).
AFAIK the current FP3 dev and stable versions are to be flashed via fastboot (well, there’s a shell script doing this) and not applied via adb sideload (was different in the past).