Just chiming in to say that I am also experiencing this problem. TOptional’s in my project are all marked as incomplete type "TOptional<T>" is not allowed", but my program compiles without errors and appears to run correctly.
The usual fixes for intellisense problems I have had in the past did not solve this problem: closing and reopening the visual studio solution, deleting the .vs file in the project directory, regenerating the visual studio solution; none of these fixed the issue.
I have only experienced this intellisense problem with TOptional.
July 13 2025 Edit: This problem is still present in Unreal Engine 5.6.0 with the latest Visual Studio Community version (17.14.8)