It doesn't happen every time. Yesterday I couldn't reproduce the problem. It seems the problem tends to happen if the computer has been turned off for hours... i.e. this morning (after a full shutdown) it booted straight to BIOS, this time I kept my eye on the prize and it literally just booted into BIOS and I hit F10 (no changes to save) and it booted into windows like normal. When I try restarting or just shut down and power it back up within a minuted I haven't had it boot straight into BIOS yet.LOL! You're troubleshooting here!...Keep your eyes on the screen.
Maybe a read through this might help https://www.bleepingcomputer.com/forums/t/708317/windows-10-wont-boot-off-ssd-after-attaching-hdd/