You should ground the MOSI pin to have a defined input.
I count more than 12 clock cycles on your picture. Sure you do not have 15 bits to transfer?
Yes, I'm sure. That is the only way this particular master communicates. 1'st is for sync (sadly, I don't know how to make SPI start from the 2'nd), and the other ones are error and parity bits.
I will try grounding MOSI, thank you
So you need to increase the slave's width to 15.
SPI works as:
For every bit the SPIS interface gets, one bit is returned immediately. When the very first byte is sent, the interface usually does not "know" yet what to answer, so a dummy byte is returned which should be skipped.
SPI has no read command, so you must send dummy bytes to retrieve the information wanted.
There is no sync or parity defined for SPI, you need to handle those in software.