Sitewide Accessibility not storing

I have Sitewide Accessibility set as Enable and Remember User For set as 12 Hours, yet nothing is being remembered.

As soon as the user switches page they have to reenable their preferred accessibility option, even if revisiting a page they’ve already set their preferred accessibility option on.

Unfortunately, I cannot at this time provide a URL publicly as the website is still in development, but wondered if anybody else had come across the same issue and whether there was anything I needed to do/change/check.