I recommend checking your boundary conditions . Can you please share the set up screenshots for further information. I am attaching a best practice video link for dynamic mesh of piston similar to your case so you check the dynamic mesh settings.
(40) ANSYS Fluent: Dynamic Mesh Problem for a Piston and Reed Valve Part II - YouTube
(40) ANSYS Fluent: Dynamic Mesh Problem for a Piston and Reed Valve - Part I - YouTube
I hope this helps you solve the problem.
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