Please see the following link for the schematic of the IAR development board. I am using SPI3 on pins 111, 112, and 113. There are pullup resistors to 3.3V on the SCLK line. SPI3_MOSI pin 113 drive the SDIN while SPI3_SCK pin 111 drives the SCLK of the DAC7811. The SYNC signals comes from an I/O Port pin 58 under software control. The SPI interface is programmed for high-level idle state and data samples on first falling edge.
https://old.iar.com/website1/50.0.1.0/581/STM32F407ZG-board-schematic.pdf
I noticed on your scope capture you have 17 clock pulses while the STM32 SPI interface has 16 clock pulses but it met all the DAC7811 timing requirements. What is your clock frequency?
Thanks