This is what drives me crazy about WordPress, every plugin blames the theme or another plugin and noone ever seems to want to fix an issue.
I have found a workaround for this so I’m not fussed if it gets fixed or not. But as I mentioned, this issue also occurs on the standard checkout (have tested), it has nothing to do with my theme, and it previoously worked perfectly.
I have also tested turning off all the plugins, and the plugin which causes the issue is the “User Switching” plugin.
