Can't boot into windows through startup disk

[johnnymichael]

I had a question in regards to bootcamp. I installed window 10 on a single drive using bootcamp a few days ago and I can boot into it by pressing the option key during start up. I just bought the RX 580 GPU (no boot screen) and I tried booting by selecting the windows drive from mac startup disk and have it boot into windows but it keeps leading me to "No bootable device found...". Strangely, with my mac graphics card and pressing option I can see that there is a windows bootable drive and I can get into windows that way but it doesn't seem to recognise it if I do it from "startup disk". Why would it recognise it through pressing option during boot but not through target disk? Thanks ahead of time!

Running High Sierra 10.13.6
Mac Pro 2009 4,1 flashed to 5,1
Windows drive is formatted to NTSF and its EFI boot