As reported on the German-language Wikipedia's feedback page, there are still (again?) many cases in which the VisualEditor adds nowiki tags around ISBNs, such as this edit.

All cases can be found in this abusefilter log (note that some of the edits there are cases where a user edits an article which already contains an ISBN enclosed in nowiki).

This problem has come up in the past and was resolved with T117165: Sometimes user-inserted ISBNs don't get converted to magic links somehow, meaning Parsoid has to nowiki them…, so perhaps we are dealing with an incomplete fix or a regression here.