With nuget client 3.5.0 the support of semver 2.0 start to be supported (in a limited way)
see doc here : https://github.com/NuGet/Home/wiki/SemVer-2.0.0-support
Semver 2.0 allows to have this format for nuget versioning : Major.Minor-beta.X (where X is a number).
For now if you have same package with following versions : 0.4.2-beta.2 and 0.4.2-beta.1 Artifactory will only find one with nuget api
Orange has sent me some packages to reproduce the error (so if you need them, just ask)
IMO more people will move to this semver 2.0 during Q1 2017 and we should be ready for it (specially when nuget.org will start to support it making our remote repo partially unusable on it)