Fixed it. Incase it’s useful for anyone else, there was a clash with the Woocommerce Stripe Payment Gateway. Disabling ‘Enable Payment Request Buttons’ fixed my issue.
The backgrounds on my site are still behaving weirdly, but only over a certain browser height. Eg, full screen-browser on my 1920×1080 monitor. Luckily it’s not interfering with the usability of the site but it is frustrating!