One of the reasons to get IO Error (IONFS_EIO = -6) is when FFS is not able to communicate properly with flash device. Since you are observing this issue only after power up, I suspect you are not waiting enough before accessing the flash. Please see power up and power down requirements in datasheet, https://www.cypress.com/file/196886/download page 24. I have pasted the screenshot below.
Please make sure that you are waiting for more than 10ms before trying to mount the file system from flash device.
Also, please let me know, if you are trying to perform any other operations to flash device before trying to mount it after power up?
Thanks and Regards,