As reported in the PR, 15.0-BETA1 bootonly installs stable/15 packages. Use base_release_# for BETA and RC as well.
@ivy may take on a better rework of this (coordination between release/ and usr.sbin/bsdinstall/) and I have some plans to rework information stored in newvers.sh, but this is proposed as a straightforward stopgap for 15.0. I'll defer to a nicer approach if available.
PR: 290238