Give it time to adjust. What you are experiencing is the normal variation after being promoted to a new tier. Basically your average in the Amateur tier is no longer a factor. At the beginning of a new tier, your average will immediately adjusted, noticeably, for play at that level...you'll notice greater movement at this time as higher scores will affect the algorithm. Eventually things level out...and, as I said, averages begin to move downward slowly, and "a bad day won't matter". Below is a cut and paste from the FAQ Home section from over there------->
Joey
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What is my Average Score?
Your Average Score will be based on multiple
best scores within ONLY your current Tier, not scores that you shot when
in previous Tiers, which better reflects your current skill competing
with the tee boxes and green speeds in your current Tier. Your Average
Score will be based on ONLY your best scores, so completing a bad round
will not negatively impact your Average Score or Tier. So be sure to
finish all your rounds and gain the extra experience points, as a bad
day won’t matter.