Ian Jackson wrote:

I forgot to mention that, in the LSB or USB positions, my TS520s also
seems to work on SSTV (and morse).

As well it should. In the LSB and USB positions, the BFO is turned on.
SSTV is essentially sideband (requiring the BFO), in that the video SSTV
signal is plugged into the mic jack...and naturally, CW (morse) requires
use of the BFO --so what you describe is not surprising.