Issue with video mode on XT-30 II

Hello,

I have an issue with video mode on my camera. I got the Fujinon 70-300 f/4.5-6

I shoot on .mov format, 4K@30FPS. Whether with low or high SS, when I move around while recording, let's say tracking a Pelican at 300mm, it's like the subject on the image accelerates then slows down, like if a stabilization was processing but not efficiently.

Where does this come from, and how to fix it ?

Thank you for reading