CONFIG_FEATURE_MOUNT_LABEL enables UUID/LABEL mounts. CONFIG_FEATURE_MOUNT_FSTAB enables fstab and mount -a. But these options do not play together. Enabling either of them or both, does not let you to use LABEL/UUID in fstab.
Ironically, I reported the same bug to util-linux and it was fixed: https://github.com/util-linux/util-linux/issues/1492 I wonder if busybox's mount also uses libblkid from util-linux, so maybe in busybox this is now fixed too? Or the same bug in an entirely different code bases?
From my understanding, this is a different code base: https://git.busybox.net/busybox/tree/util-linux