Main class outside project not found

[kmartin88]

Describe the bug
I am using STS4 for Eclipse and I have a project in my workspace which uses a custom framework on top of Spring Boot. The main class is not contained in the project directly but is located inside this custom framework.
The project shows up in the boot dashboard, even if I check "Hide local non-runnable apps" in the filter.
However, the default run configuration created by the Boot Dashboard does not contain a main class which leads to "Error: Could not find or load main class".
When I edit the run configuration (click on Open Config in the context menu) and then click on the "Search..." button next to "Main type" the main class doesn't show up.
It works perfectly fine if I just put in the fully qualified name of the main class myself. It just doesn't seem to be able to find the class by itself.

Is this a bug or intended behaviour (meaning does it only look inside the project sources for a main class instead of the whole class path)?
In the case of a bug I can try to build a little sample to reproduce it as I can't show you the original because it's a customer project.