While testing the mod on a creative world to see if it worked, while the Wither Storm was at the end of phase 6 before moving on to phase 7, the game crashed and gave me this message:

The game crashed whilst ticking entity
Error: java.lang.StackOverflowError: Ticking entity
Error code: -1

Here is the full bug report:

crash-2022-06-11_22.43.07-server.txt

And if you ask for the list of mods, I only had the Wither Storm mod installed.

Should be fixed in the next upcoming update