Hello @Seppia89, how are you?
I’ve been testing what you mentioned (UE 5.4.2), and it seems that the “Affects Indirect Lighting While Hidden” checkbox doesn’t work properly with Spline Mesh Components—but it does work fine with Static Mesh Components.
A possible workaround is to convert your splines to static meshes, especially if your goal is just to use them for lighting and not render any actual geometry. There’s a built-in tool for this in the editor, and it also helps optimize by merging everything into a single mesh. Here’s a video showing how to use it:
https://www.youtube.com/watch?v=Tr8SlLqX2XM
Helpful links:
Lumen & Emissive Materials: Using the Emissive Material Input in Unreal Engine | Unreal Engine 5.5 Documentation | Epic Developer Community
About Affects Indirect Lighting While Hidden: Emissive material only visible to Lumen (invisible in game)
Hope this info helps! Cheers 