Pixel XL Fastboot Works to Flash Stock ROM+TWRP. Unseen as USB in TWRP or OS

[Crafter_Laughter]

I have 3 of these devices I'd successfully flashed TWRP to months ago. I can plug the other 2 into any of my Win10 PCs and they are functional as MTP devices in TWRP or the OS. Something is wrong with this device in that fastboot works fine, yet it is not recognized as a USB device at all in Windows. No device detected ding. No yellow exclamation point in device manager. Nothing in Nirsoft's USB DevView. I was unsure if this device received an OTA update so I went ahead and flashed the latest stock firmware as of 5/24/2017

It successfully flashed albeit unusually slow (even though Nirsoft USBDevView showed it connected at USB 2.0 and not 1.1). It also successfully flashed the latest Pixel XL Marlin-3.1.0-0RC2
Even if I attempt to toggle MTP in TWRP there is no response in Windows (on any of my PCs) nor is there any response when it's booted to Android 7.1.2. Same when toggled to PTP. Same w/ USB Debugging enabled. The device charges fine. Checking OTA updates shows none available. Factory reset option in bootloader didn't help.

I'm sure I'm missing something like I somehow have the incorrect firmware loaded for something. Anyone know what specifically is responsible for the USB at the firmware level that would allow it to function in fastboot mode yet fail to detect when booted TWRP or OS???