[BUG] purls don't resolve

[Hannah-Doerpholz]

Describe the question or discussion

I checked out your ontology on OLS4 and noticed that the purls don't resolve, or rather, they lead to "term not found" on Ontobee. One such example is this:
http://purl.obolibrary.org/obo/MS_4000000
I'm not sure if this is something to fix from your side or from the OBO Foundry though, or even from Ontobee.