Rendering discrepancies - Dev VS Production server

This may not end up being strictly a Hugo problem, but in case anyone has seen something like this before.

I’ve built a site based on Everything is functioning as expected when running the development server. However, once on a production server, by which I mean either the Netlify build, or copying the local dist folder onto an Apache server, I get some strange behaviour. There are issues where two different scripts (Swiper and Fitty) are wrongly calculating widths of elements. Sometimes this only happens when refreshing the page, and most prominently in Firefox. There are no errors, it’s just as if the scripts are firing before the layout is known. In both instances, resizing the browser triggers a recalculation, and everything is rendered correctly. The JS files are identical in both environments.

Anyone have any idea what is happening here?