ADB doesn't recognize phone in bootloader

I'm having a weird issue.

When my phone is turned on, ADB recognizes it just fine. But in bootloader mode, it can't find it.

When in bootloader mode, it's listed as Android Bootloader Interface in the device manager, but adb devices shows nothing. Curiosly, fastboot devices does show my phone.

It's a new PC, with Windows 10 Pro Anniversary Update x64 and the latest Android SDK (platform-tools and driver). I've done this a million times, first time I have an issue.

PS. It happens in Nougat (NRD90M) & Marshmallow (MTC20F).
PS2. I've tried different ports. Also tried removing and uninstalling everything (with USBDeview) and starting fresh. No luck.

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