strange operations Smart SDR

This should depend on what kind of SSTV you are using. Analogue SSTV, such as that used in the MMSSTV software, must use USB and I have never seen a guide claim that DIGU is suitable. Analogue SSTV does SSB modulation on a preprocessed SSTV FM Sub-carrier.

If using Digital SSTV with for example EasyPal or KG-STV then the radio is in DIGU mode because it uses a digital protocol called mimimum shift keying.

I use both types of SSTV with the software named above and have done for years. Maybe something has changed in the interim regarding the guides and the radio mode in use. I somehow doubt it but I'm not an expert.