Hello nl13286
Yes the SCU-17 works fine with both my FT-891 and FT-817 with QSSTV (9.4.4) on Linux. I use Linux Mint (20.3 Una) and PiOS (Bullseye) depending on hardware.
You shouldn't be seeing.the FSK indicator during transmission.
I can do some screenshots of the Sound and CAT configuration if you need it.
73 Glen
Glen - G0SBN