Hi,
Apologies for the delay. It’s a known issue on Epic’s end that instancing doesn’t support spline meshes. After testing, I noticed that Approx had the same effect.
I’m seeing if I can get the JIRA that I could share with you, or if I can create a separate bug report for this specifically for approx.
In the meantime, a workaround that might work for you would be merging the spline into a static mesh actor which can be done by selecting your spline mesh > clicking on “Actor” at the top toolbar > Merge Actors > Merge > Saving it and then using said static mesh.
Edit: Unfortunately we don’t have a tracking link, but Epic mentioned this case in the JIRA with instancing.
Regards