Just to confirm, I guess by CyPmSleep2, you mean CyPmSleep.
Also are you sure if the device is moving into the sleep mode and not stuck in the while (!(CY_GET_REG8(CYREG_PICU15_INTSTAT) & 0x40)); loop.
Can you please attach you project, we can have a look at it. This will help us to understand why changing the UART interrupt priority causes the code to reach the last USBFS_Resume.