WGT's FAQ's on the right state:
"Advancing to the next Tier requires a qualifying
Average Score AND a minimum amount of completed stroke play games,
which will take a little more time but will help players reach the
Levels necessary to unlock upgraded equipment that is helpful to compete
at higher Tiers"
and
"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."
I hope that helps.