It seems like you are experiencing a mesh deformation issue in Spine's Preview mode that doesn't appear in the Animate mode. This could be due to several reasons, such as incorrect settings or a bug. First, ensure that the "Deformed" checkbox is checked in the Edit mode, as this can affect how deformations are displayed. Additionally, check if there are any deformation keys set in the dopesheet that might be affecting the mesh. If the problem persists, it might be helpful to verify that your Spine project and runtime versions are aligned, as mismatches can sometimes cause unexpected behavior.

If these steps do not resolve the issue, consider sending your Spine project file to contact@esotericsoftware.com for further investigation.

acszsolti I have watched the video you uploaded, but unfortunately it only shows the preview, and it is quite unclear what you are having trouble with. We would appreciate it if you could show us both the expected state and the unexpected state (the state in which the problem occurs) when describing the problem.

It's hard to pinpoint the cause of this problem without seeing more details about your skeleton setup, but if you can't elaborate on the forum for confidentiality reasons, please email us a Spine project that confirms the problem: contact@esotericsoftware.com
Please include the URL of this forum thread in the email so we know the context. Then we can take a look at what's wrong.

Does the problem still show if you add keys on scale and rotation at the beginning and at the end of your animation?
I coundn't notice anything problematic from your video, but I also can't explore your file to inspect it better. Are you using the latest version of Spine?