Tested on 6.4.1 (1669)

Steps:

1. Go to an article with ISBN numbers listed in the references or footnotes (Example article: Basic Income, reference #6)
2. Tap the ISBN number

Expected:
The Book Sources page appears

Actual:
The page you specified does not exist error message appears

Note - The same error appears in the preview windows if one long-presses an ISBN link on iOS 13