I know that every so often you get bumped down two ranks, but how often is that? Is it every month or at the end of every Battle Series?
It is at the end of each season that your rank will decrease. When it decreases, it goes back down to the basic rank, or Beginner Tier.
Source: https://www.ign.com/wikis/pokemon-sword- shield/Battle_Stadium#Casual_Battles