Boot up failed on Musca-A2 with PS and ITS using real flash if the flash is not erased properly. The boot is successful and the tests pass if all the associated flash areas are erased (SST/ITS/NV-counter flash areas) areas. The following command was used to erase the flash areas :

srec_cat bl2/ext/mcuboot/mcuboot.bin -Binary -offset 0x200000 tfm_sign.bin -Binary -offset 0x220000 -fill 0xFF 0x420000 0x425000 -o tfm.hex -Intel

The current theory is that FS data associated with a previous version of SST might have been present in the flash, and caused incompatibilities with the current version. But possibly the SST module needs to be hardened against such issues and needs further investigation.