I turned Legend in Sep 2018 and my average went up to almost... well very high... before coming down to about 65.9 after about 580 rr. It had never gone up after 500 rr again until very recently (last update) it did start to climb after bad rounds!
Either the number required has been increased from 500 to - wild guess: 750
WGT has fixed what has always been a mistake in that par3, par5 and custom courses don‘t influence average but were counted as rr towards saturation anyway, which is not logical.
I shall find out if the latter is true in about 43 proper rr when I‘ll reach 500 of those.
Now they just need to take those „non-counting rounds“ out of the players stats as well.
And of course introducing player stats since last tier change would be nice but I believe this request has been posted many times and is falling on deaf ears.