Stable should work fine on iOS 12 - it was cut before we started using replaceAll in the critical path, so the polyfill should not be required. The issue in the OP doesn’t look like it would be caused by a missing library.
More likely it’s a theme/plugin introducing the issue. @orangeandblack5 can you try in Safe Mode?
Also, any chance you can share a link to the forum?
(BTW I re-uploaded the video in the OP so there’s no need to visit wetransfer.)