@mararm Actually, the problem is deeper: GNU Social really cannot provide information on a "Remote user" (a User fromanother server). So in GNU Social this function works for local users only.
If you have accounts on different servers, you may have noticed that #AndStatus understands that users on different servers with the same webfinger ID (like email) are actually the same User. This is why a profile shows notes from different servers ("Social networks") and you can switch between profiles of the User as shown at different social networks. E.g. I can see your profile (and posts) via 6 social networks (some of them are down now...) Profile at each Social Network can have different avatar... https://loadaverage.org/attachment/4930783