Split artist does not work if an artist has been renamed. (See attached related ticket for AC issue). For example, create a "Tom Foo / Cindy Bar". Now rename it to "Tom Foo & Cindy Bar". Because of the related ticket, all "Tom Foo & Cindy Bar" entities are still set to AC "Tom Foo / Cindy Bar".

"Tom Foo & Cindy Bar" can now not be split (except in some cases where, if I rename every single instance of AC "Tom Foo / Cindy Bar" to "Tom Foo & Cindy Bar", it sometimes will now work). When you try, split complains that "Tom Foo & Cindy Bar" has nothing associated with it, even when you're staring at a screen full of releases/etc.

You can only split "Tom Foo & Cindy Bar" if you rename it back to that original name of "Tom Foo / Cindy Bar", where it now matches the associated entities' ACs.