Unity C# Intellisense not working

I've got the mono5 & dotnet sdk's installed and Unity and VSCode detect them but I'm not getting any code completion (intellisense) has anyone encountered this issue and found a solution?

Edit: I've made a matching issue in the Unity Flatpak repo just in case it falls on them, will close the incorrectly placed one once I know who it falls on. And will close the other once resolved or deemed unfix-able.

Edit 2: I've narrowed it down to because there's no sln/csproj files being created. And I see the script in the Unity VSCode package that's supposed to do it but nothing is happening.