The status of Weather Ball in Dynamax should be based on the type of weather change
Become Max Flare on a sunny day
Become Max Geyser on a rainy day
Become Max Hailstormr on a hail
Become Max Rockfall on a sandstorm

Thanks for the report, I've been able to confirm this.

Hi there, sorry for the delay! We're currently working through older tickets, and upon testing this issue, I can see this is still occuring in the current version (8.3.6).

My testing method:
1) Gave myself a Venusaur with the move Weather Ball. Gave Venusaur 10 Dynamax candies, and equipped a dynamax band to myself.
2) Set the weather to rain in game.
3) Engaged in battle and dynamaxed, using Weather Ball (which, as is normal, becomes Max Strike).
4) According to Bulbapedia, this move should say "Max Strike" but instead do whatever actions the corrosponding weather dictates - as it was raining in game, the move should have had the effects of "Max Geyser"(creating rain for 5 turns), but instead had only the effects of Max Strike (does damage and decreses the enemy's speed).

As such, I'll move this issue to internal; thank you!

Because Pixelmon battle weather is not tied to Minecraft weather as it is in the Pokemon games, setting the Minecraft weather will not cause battle weather to be set to rainy - another move or ability will be needed to cause Max Strike (Weather Ball) to take on the proper elemental effects, but the fix in the upcoming version now properly causes max geyser when the battle-weather is set to rain; max flare when sunny; etc.