Chilling a wet target will freeze them, but casting Rain on a chilled target will not.
Some patches of fire can last through rain, though campfires should generally go out.
If you create a new profile and start a new game on Explorer difficulty (with a custom wizard to change Boulder bash to Rain, taking an ability point from Willpower or Loremaster), does Rain work then? You can have the wizard can Flare very close, and then Rain, or in the first fight have a fighter throw a firestorm grenade and walk into it. What about the burning ground in the tutorial cave, or the ship in the Cyseal harbour?
Try verifying local files: in the Steam library, right click on Divinity Original Sin Enhanced Edition and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Cache...' button.
With the GOG version, in the (optional) Galaxy client, select Divinity: Original Sin Enhanced Edition in the left column, then click on the More button, and in the Manage menu select 'Verify / Repair'.
