When you add a valid ISBN number using the "magic word syntax", VisualEditor doesn't automatically link it (T54204).

However, existing ISBN numbers are formatted with links to Special:BookSources. This inconsistent behavior is confusing and leads to errors (as in T63558: VisualEditor: ISBN link labels (from magic words) can be changed, but not the underlying links).

I'm not sure what VE should do, but it should behave consistently with regard to all ISBN items.