Roland’s documentation is not good but it is likely that the BANK and SBNK settings are not necessary. Because the System-1 only has 64 program memories, you can probably set those using only values of PROG from 1 to 64. Edit to correct: it seems my guess is wrong; I read on the web that you need to use SBNK in the range from 1 to 8 and then PROG in the range 1 to 8 to access the 64 locations. BANK should have no effect.
The P12 should not respond to BANK, only to SBNK and only in the range from 1 to 4.
Try these values in case they work. If not, we can investigate further.