Strange "shrinking" animation on website header when scrolling up

Whenever I scroll up on my website on the mobile site, I get a strange "expanding" animation when I scroll up that looks quite unnatural. This has always appeared on my website in the past year or two that I've had it. Is there any way to get rid of this animation? Attached is a video of the effect I'm seeing. 

Note: seeing animation style site-wide to "None" doesn't fix this issue.