VisualEditor encloses ISBN numbers inside <nowiki> tags.
Try the following steps:
- edit a page with VisualEditor
- add any ISBN number but don't c&p it!
- click "Save changes"
- click "Review your changes" to verify your edit and save it
You'll find the ISBN no enclosed in <nowiki> tags like <nowiki>ISBN 978-3-9815841-5-8</nowiki>
See [1] for a sample
On de-wiki there is a abuse filter [2] now but you are unable to remove the tags manually
The tag filter says this edit is made by VisualEditor [3]
[1] https://de.wikipedia.org/w/index.php?title=Benutzer%3AXqt2FTest&type=revision&diff=158437087&oldid=157652172 for a sample
[2] https://de.wikipedia.org/w/index.php?title=Spezial:Missbrauchsfilter-Logbuch&wpSearchFilter=236
[3] https://de.wikipedia.org/w/index.php?title=Spezial:Letzte_%C3%84nderungen&tagfilter=ISBN