I thought there was a flood every 500 rank until 6500. I was at 5000 and dropped below with a loss after patch.
Here is the excerpt from https://playhearthstone.com/en-us/news/23523064
There are rating “floors” between 2000 to 6000 for external rating. Your external rating does not drop below each “floor” once it’s above it. Currently there is a floor every 500 rating: 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000.