The game will fail to switch artifacts when one of those occurs:
- You change from a spec with an acquired artifact to one without (the previous artifact remains equipped)
- You change from a spec without an acquired artifact to one with (the previous weapon remains equipped)
- You change between specs with an acquired artifact and the artifact of the spec you changed to is in the bank (the previous artifact remains equipped)
so basically it only changes between two artifacts if both are directly available (speak either equipped or in the bags).
Also if you got yourself in a state where you have the wrong artifact equipped, the return of C_ArtifactUI.GetNumObtainedArtifacts() does not account for the inactive equipped artifact. It appears Blizzard scans your complete inventory (including the bank) when you call C_ArtifactUI.GetNumObtainedArtifacts() but excludes the items you cannot use
