When I launch MC20 I see the MC20 icon on my application dock. The bar at the bottom of the Mac screen that has all the app icons. After the app starts I see the MC20 welcome screen but then I begin seeing an additional MC20 icon on the application bar. The second icon bounces up and down like the icons do when you start an app and then the second icon closes. Immediately afterward another MC20 icon appears. This just keeps going on and on.
This is how MC behaves on Macs while it is building thumbnails for files. It will happen until the thumbs in the current view are finished building.
MC19 doesn't do it because its thumbnail cache is finished (except for new files). If you want to make it do it beforehand, go to:
Tools > Options > Tree & View > Thumbnails > Build missing thumbnailsAnd let it run. It'll probably take a while, so kick it off and go to bed. Otherwise, it can safely be ignored.