Round cap disappears in 3D

[RunForestRun]

I'm animating a stroke path in AE for a straight mouth to a smile. When the layer is set to 3D the rounded cap is removed on the first keyframe but not on the second. Any ideas why this is? Or is there a better way to create this smile effect?

Thank you!

  

[Rick Gerard · Accepted solution]

I cannot reproduce your problem. Try redoing the path from scratch with the layer already set to 3D.

[Kyle Hamrick]

Rick's suggestion would get you to the same place, but I think you may have accidentally created a closed path on the second keyframe. The 3D aspect shouldn't be a factor here. 

[RunForestRun]

Thank you for the help. I remade the path which seemed to solve it. It didn't look like there was a closed path there but I do find paths trickier to use than in Illustrator.