Updating an old site where classes are applied to nav li items via the backend custom classes.

When I add any new items and classes, the classes are saved. BUT the classes are not showing when inspecting the front end.

I’ve cleared browser caches and not using any plugin caching etc.

Strangely, if I edit an existing menu item class, that also will not show. But if I change it back to the original class, its visible again on the front end.

I’ve also updated to 5.2.4 and then gone back to 5.2.3 just in case it was a new WP problem, but nothing changes.

Any help on this is much appreciated!