Bootcamp: My Mac Only Boots into macOS No Matter What Partition I Choose
AFAIK High Sierra already "introduced" APFS. The Bootcamp partition is formatted NTFS, I thought. If you set the startup disk to macOS in Settings (while in macOS), then, when you startup holding the Alt key, the boot window should let you choose between the two partitions. I cannot check now since I am a long way from base.
I am not sure when Startup disk is set to the Bootcamp partition, everything will work correctly as expected.