Moto G4 goes into fastboot by default

[DRX777]

Got a Moto G 4th Generation, the Amazon subsidised model XT1625. I flashed it to stock Moto G4 OEM when I bought it, but was too late to get a bootloader unlock code. After they updated it to restore all the Amazon bloatware, I attempted to flash it back to OEM, but failed. Gave up and decided to live with it. Got another phone on the way now, so I wanted to trade in this one; buuut. They won't take it, because for some reason, whenever it is powered off and on, it goes into the fastboot menu. Even after a factory reset (which features a stop on the fastboot menu, as a factory reset involves a power off and on!) this situation persists.

I guess at some stage of attempting the OEM flash, I changed some configuration somewhere to make fastboot the default boot mode. How do I go about undoing that, so that it just does the default boot up sequence, and fastboot goes back to being "hold volume down" to get into?