Hi marvide
I seem to have missed this thread when I was mentioned before. Apologies for that.
Are you using OAuth 2.0 (passing Authorization and client-id headers) or a REST V1 Session Key (passing li-api-session-key header or restapi.session_key parameter) to authenticate to the API when you make the signout call? Also, are you making the signout call as the same user you are signing out, or a different user?
It looks like if you authenticate via OAuth 2.0 it signs off your OAuth user (revokes your access token), but doesn't sign you out of any web sessions you have going.
-Doug