On Xbox Series-S, Minecraft Bedrock, Realm, Hard mode, beginning with 1.20.10 update. 

For a couple years I have built an obsidian platform one block below sea level in an ocean to fight the Wither. As soon as 1.20.10 dropped, the following happened which had never happened before:

1)  I spawn the Wither successfully.
2)  I fight the Wither using a bow and arrows until the Wither reaches phase 2 (shakes, glows, explodes, drops wither skeletons).
3)  As soon as the Wither explodes and drops the wither skeletons, the Wither shoots straight down through the water and punches a perfect 8x8 column down through the water and through the ocean floor until it reaches an open cave system approximately 40-60 blocks below sea level (y=0 to y=20).
4)  Once it finds the cave system, it begins destroying all available blocks causing the realm to begin lagging. 

As I mentioned, I have used this method successfully for a couple of years with no issue. As soon as 1.20.10 dropped the Wither Boss' behavior changed, making it exponentially more difficult to fight the Wither Boss without causing realm-wide lag.

Bug or feature?

Thank you!!