Hello Santtu,
What you are attempting to do is exactly what your project is achieving. By telling your object to consistently switch back and forth between the consistent static mesh and attempting toggle its visibility you will definitely get a flickering. This is not a bug, but just a logical error. I do understand that you are attempting to optimize, but with something this simple you are actually making it a bit more difficult than it needs to be.
If you are attempting to have a smooth transition between your static mesh and the morph target you need to establish a timeline with a SIN wave. I actually have a great reference to a question very similar to yours. In your case the only thing you would need to change would be your length of time of your Timeline to 4.0 seconds. Here are a few images of your own project to help you get started. This smoothly transitions between your morph and the static mesh.
Related Question:
Blueprint Timeline SetUp
Timeline SIN Wave SetUp
I hope this gets you on the right track!
Thank you,