We are going to check on this and will get back to you soon.
Some BIOS implementations may have option for additional timeout at the end of the POST (e.g. in BIOS for PCSD server motherboards of Grantley generation we have option " System Boot Timeout " in " Boot Maintenance Manager " tab in BIOS setup) - This allows additional time out for initialization for certain devices. You may need to check with ASUS if they have this.
Alternatively, it may happen that for some reason there is not enough power for the Intel® SSD DC P3700 Series to start up because there may be many devices starting up simultaneously. You may try enabling staggered spin up on HDDs if there are many connected and if such option is available in BIOS.
Could you please check if clearing CMOS without drive re-installation to different PCIe* slot can help? If so, it may indicate BIOS issue which needs to be investigated by ASUS.
When you are cold booting the board and the Intel® SSD DC P3700 Series is not seen in BIOS, can you boot to EFI shell (if possible on this particular motherboard) and list all PCIe* devices by pci command? It will show if board recognizes the P3700 or not.
It may happen that the P3700 is in PCI devices list but not showing as bootable device which is a different situation. We also wonder if you could check this driver in a different motherboard to confirm it is healthy.