Contains filter on Microsoft Graph query not working

The contains function is not available for users. startsWith is available though.
e.g. https://graph.microsoft.com/v1.0/users?$filter=startswith(displayName,'Garth').

Additionally you can try the people API (this is only available on /beta). This supports $search AND will do fuzzy and phonetic matching. https://graph.microsoft.com/beta/me/people?$search=Garth