Was there a change that broke Visual Studio Code's intellisense

I've been using Visual Studio Code with the Legacy C# extension, ever since external editor support was built into Unity. Things were working great.

It seems like after I imported a fairly large Asset from the asset store the editor no longer knows what's up throughout my classes anymore. But, I'm not sure if it was importing the asset or whether it was updating Visual Studio Code, or something else which caused the breaking behaviour.

Essentially if I have a public method in another file, Visual Studio Code no longer lets me know it's there. It also doesn't warn me if I'm using a method which doesn't exist. Nor does it warn me if I have unused variables, and so on, like it used to.

Instead I'm forced to refer to the Unity console.

I wish I know which change broke things, I'm asking here in case it's a wide spread problem and not something specifically I did.