OK. Will try this on the week end. DECODEMODE was not implemented as it does not relate to the LED matrices, but easy enough to add it into the control switch and it can't hurt.
The datasheet is fairly clear on what happens on reset:
On initial power-up, all control registers are reset, the display is blanked, and the MAX7219/MAX7221 enter shutdown mode. Program the display driver prior to display use. Otherwise, it will initially be set to scan one digit, it will not decode data in the data registers, and the intensity register will be set to its minimum value.
but I guess these clone chips don't conform.