Compiles, but intellisense complains about a third party lib

I have a third party lib file that I am linking against. Compilation works fine, but intellisense does not list member functions etc, nor does it autocomplete. I have added 2 other third party libraries before in the same project, but this time intellisense is freaking out.

I have double checked the path. I have restarted my PC. I have redone every step and the same happens. I tried cleaning and rebuilding.

The only thing I can think of is that the folder which I am giving the path to contains a lot of subfolders where the headerfiles are. Can this be a problem?
Anyone know why this is happening? It is mighty irritating and a hindrance to my workflow.

Thank you for your help.

EDIT:
Here is the line of code I wrote:
PublicIncludePaths.Add(Path.Combine(ThirdPartyPath, "opencv", "build", "include"));

The path on windows is “$(project_folder)\ThirdParty\opencv\build”

Somehow changing the mode from DebugGame to DebugGame Editor and then back fixed it.