Alexiss8: would his average start over at 150 or does the starting average change as ranked rounds do?
There are two "regular" states of average in the situation of tiering up:
1) The last average from the former tier which is shown immediately after the promotion, together with the new tier title - e.g. Master ave. 67.00,
2) the first average of the new tier, equalling the first score since the calculation has been reset. E.g., Master ave. 72.00 after playing a 36 over 9 holes.
A 150 average is special, either corresponding to the improbable nine hole score of 75, or from scratch, i.e., no scores have ever been played or recorded by the system.
I suppose that this player has been playing "mobile" games exclusively, never finished a ranked strokeplay round, and was promoted in the mobile game thanks to his levels.
"Facebook" accounts may also show a 150 average but I don't know details of these.