Most often and especially on mobile we find players who only play quick play multiplayer rounds which keeps their average from changing as they have not logged any ranked rounds.
Sometimes the answer is more sinister and a player creates an account and intentionally keeps it at 150 to make themselves seem like a worse player than they are for a competitive advantage.
Are you sure they were a level 12 Tour Pro? That sounds impossible. If you remember their username and wish to report them, you can privately message me with their info.
_CHAMPION