I think this is known and intentional (as thanks relates to users). I
personally feel the icons are sufficiently different although maybe
both having a smile is the cause of conflict. Designers what is your
opinion?
The smiley icon makes perfect sense for thanks, since we're trying to convey positive communication. It doesn't make sense to convey a smile on a generic "registered user" icon I think, and is unusual. On most mobile experiences I am used to (Dropbox, Spotify, Snapchat, Instagram, Gmail, Twitter, and more) users are represented either by a neutral emotion human face or an avatar. Adding a positive emotion for no functional reason on the registered user icon feels gimmicky, and like a waste of a design tool that we can and should be using to increase positivity around actions.