I am using the latest version of all the mods 6 (1.5.2), and botania version 1.16.4-411, and mythicbotany version 1.6.3-1.2.4.

In the last update from all the mods 1.5.1 to 1.5.2, the raindeltias stopped working during normal rain and now only work during thunder storms.

It is intended that the raindeletia only produces mana during a thunderstorm if not placed on vivid grass or enchanted soil. In the current version it also won't generate mana on vivid grass, I am going upload a fix to curseforge very soon.

Oh cool. The mythic botany section of the guidebook still says it generates in rain and even more in thunderstorms, which is why I was confused.