It sounds like you’re adding automation to the parameter by right-clicking on it in the transport bar of the “Weather” event. This adds automation to the preset parameter, and so affects every parameter based on that preset parameter throughout your project.
To add automation to only the parameter value associated with a specific event instrument, select that event instrument so that its parameter knobs are displayed in the deck, then right-click on the knob of the parameter you want to automate in the deck.
Without being able to inspect the event in question, I can’t be sure why this happened.
That being said, there are a couple of common reasons why a parameter knob might not appear in the deck of an event instrument. Built-in parameters are continually updated by your game’s code, and so cannot be usefully set or automated on a per-instrument basis; and global parameters can only have a single value across your entire project, and so cannot be set on a per-event instrument basis. Were two of the parameters of your nested event built-in or global?