Comment # 28 on bug 1094834 from Takashi Iwai

Unfortunately, you cannot control the release number freely when you building
on OBS.  The release number is determined by OBS checkin.

And yes, it's a known problem for long time.  It hasn't been a big issue on
Leap 42.x, so far, just because the kernel version numbers increased via stable
updates.  But for Leap 15.0, it won't happen, so this release number problem
appears more often.

So, for testing, it's advised to increase the number of kernels you keep by
editing /etc/zypp/zypp.conf.  Put more "latest-*" in the multiversion.kernels
line.