Running Visual Studio 2019 Community with Unity 2020, I regularly get loss of assembly connections.
Say asm A refers to asm B and C, opening a script from asm A could show errors for all usage of asm B while still being ok with asm C code. One occurance is opening a text file (json), it would open notepad instead of VS and shows the loss of connection in VS on opened files.
It also displays the warning in Visual studio about the project not being generated from Unity. I tried to fix that with linking the exe from Edit/Preferences/Editor but it keeps coming as it has been explained in other threads.
The solution I have to use is to add/remove an assembly reference in the asm inspector to trigger a reload. Then it will be fine for a while until it shows up again.
I already had the issue with VS 2017 and hoped the update to 2019 would fix it.