Had the same problem and can confirm it's site-wide animations.
I found simply changing the animation style (in my case, to "fade") was enough to fix the caption issue (at least for now)
For those who are lazy, site-wide animations are under site styles, or the paintbrush thingy.
https://support.squarespace.com/hc/en-us/articles/360040839591-Site-wide-animations