This is the expected behavior while playing ranked matches in Hearthstone. At the end of each season, your rank is reset to 4 ranks lower from the highest rank you achieved during the season. The player's rank is reset this way to allow for better matchmaking and fairness when it comes to skill level when the seasons reset.
Prior to the rank updates in March 2018, at the end of each season, players would start out in the new between rank 25 and rank 16. However, this logic resulted in unfair matches with experienced who achieved Legendary rank vs in-experienced players who are just starting out. The post-March 2018 update brought balance the matchmaking in order to match players who are on the same skill level.