MSI Z170 motherboard "losing boot device"

Built my new system a few weeks ago - i7-6700K, MSI Z170A-G43 Plus, 16GB Kingston HyperX DDR4. This lot went into my machine with my existing Crucial MX100 512GB SSD and WD Black 1TB HDD and R9 390 GPU.

This past few days, when powering on from cold I keep ending up in the UEFI screen after POST and it turns out that the UEFI boot BBS Priority setting has gone to "Disabled" instead of being set to the Windows boot manager on the Crucial SSD. If I set it correctly then save and exit it boots fine.

Any ideas?