SPI, wrong ~CS level after power up

after powering up the system and starting my application, the very first data transfer via SPI fails:

As you can see, the level of the SPI ~CS line, here called DL_~CS is low (=active) long before an transfer is startet.
I my software, a falling edge of the signal ~BRDY starts SPI transfers using an ISR calling Spi_ReadWrite( … ).
I the moment this call is executed, the DL~CS is set high for a moment and set back to low. This fools the slave.
All other transfers working fine.

Stopping and restarting my app didn’t show this behaviour. We need a power cycle to reproduce this issue.

With best regards