Greetings @developer0916,
Your issue here seems like the same issue another customer experienced in this thread: SPI slave mode for imx6ULL CoM
The customer in this other thread is using the IMX6ULL but this SoC uses the same ECSPI driver/framework as the IMX8M Plus. As seen in this thread the customer experienced the same phenomenon as you are here:
data sent from the slave is being received at the master, but, the data is spaced out with empty bits and takes 4 times the data to be sent from the master to receive the data from slave.
In the marked solution the customer consulted an NXP application note (AN13633) that helped them get slave-mode working properly.
Best Regards,
Jeremias