After you have issued a QSPI command from any Mailbox Client Intel® Stratix® 10 FPGA IP, wait for the command to execute successfully by reading the error code response before performing the next QSPI command. Failing to do so might trigger an error code 0x3FF.
This problem is fixed starting with the Intel® Quartus® Prime Pro/Standard Edition Software version 19.2, where you will be able to perform QSPI from multiple Mailbox Client Intel® Stratix® 10 FPGA IP without waiting for the response.