Currently APLHA packages are treated as CURRENT or STABLE versions, resulting in 13.0.s20190615125609. This version number is indeed different from the next version number but ALPHA2 would be nicer IMO.
For the BETA, PRERELEASE and RC phases the packages are versioned the same as for releases, so 11.3-BETA1 is 11.3 and so is 11.3-RC1, meaning that pkg cannot easiliy upgrade from the former the next. This happened on my Raspberry Pi which runs pkgbase.