STM32 USB Middleware: HID Output Report Transaction Error - embedded

Question

I've been having problems configuring my STM32 device to function with receiving HID Interrupt OUT transactions, where PC is host device.
I use the standard STM32 CubeMX provided USB Middleware, with edits to allow 2 endpoints, one OUT and one IN. Along with a customized HID report descriptor:
__ALIGN_BEGIN static uint8_t HID_ReportDesc[HID_REPORT_DESC_SIZE] __ALIGN_END =
{
0x06, 0x00, 0xFF, // Usage Page (Vendor Defined 0xFF00)
0x09, 0x01, // Usage (0x01)
0xA1, 0x01, // Collection (Application)
0x15, 0x00, // Logical Minimum (0)
0x26, 0xFF, 0x00, // Logical Maximum (255)
0x75, 0x08, // Report Size (8)
0x95, 0x40, // Report Count (64)
0x09, 0x02, // Usage (0x02)
0x81, 0x00, // Input (Data,Array,Abs,No Wrap,Linear,Preferred State,No Null Position)
0x95, 0x40, // Report Count (64)
0x09, 0x02, // Usage (0x02)
0x91, 0x00, // Output (Data,Array,Abs,No Wrap,Linear,Preferred State,No Null Position,Non-volatile)
0xC0, // End Collection
};
With the use of the USBD HID provided input report transaction function USBD_HID_SendReport, I can then observe the behavior of my device.
On the PC side, using USBLyzer software, I can confirm that the device is recognized by Windows, and configured accordingly, reporting back the descriptors with no errors. I then use HIDAPI to open the device, and read the reported values. This implementation works as expected, giving me communication of data from my device to the host.
However, there is no provided HID Output Report function, leaving me to implement this myself:
uint8_t USBD_HID_ReceiveReport (USBD_HandleTypeDef *pdev, uint8_t *report, uint16_t len){
USBD_HID_HandleTypeDef *hhid = (USBD_HID_HandleTypeDef*)pdev->pClassData;
if(pdev->dev_state == USBD_STATE_CONFIGURED){
if(hhid->state == HID_IDLE){
hhid->state = HID_BUSY;
USBD_LL_PrepareReceive(pdev, HID_EPOUT_ADDR, report, len);
}
}
return USBD_OK;
}
This function makes use of the same structure as the provided USBD_HID_SendReport function, instead using the USBD_LL_PrepareReceive function to prepare the EPOUT address to receive transaction, and then handling transmission through PCD.
After device initialization, I use my implemented USBD_HID_ReceiveReport function in an endless loop, being called repeatedly. I use hid_write in HIDAPI to transfer data to the device once. On debug, the device iterates through the loop, and enters the PCD endpoint transmission on every call. hid_write seemingly doesn't cause an update to my buffer as expected on the device, and USBlyzer reports a 'transaction error' on the OUT report.
Does anyone know the error in my implementation?