Hello - I don't have specific knowledge relating to how godot interacts with C#, but I do have a possible explanation.
You say it's the first time you attempt to use dynamic. It sounds to me like you're triggering the load of a library when you hit that code for the first time.
I would suggest moving your first use of dynamic somewhere that is more convenient to have the delay, perhaps at startup or during a scene transition. You can just make a trivial use of the type in order to trigger the load when you want.
Once the load is complete, things should stay loaded, and on your second use of dynamic, it should work smoothly.
I hope :).