page 447 of the manual reads that TXSSELx is asserted when bit is set to 0

isn't there a bug in the SDK's fsl_spi.c/irq handler ? see below

Shouldn't the line in blue be
tx_ctrl |= ((uint32_t)SPI_DEASSERT_ALL & ~(uint32_t)SPI_ASSERTNUM_SSEL(handle->sselNum));
This is how the code is written SPI_WriteData() and SPI_MasterTransferBlocking(). See below snapshots

