Steps to reproduce:

1. create a content type with 2 ISBN fields one for isbn 10, one for isbn 13 (in legacy or in PlatformUI, it does not matter)
2. fetch the content type thtough the REST API (or the Public API) with (replace 60 by the content type id of course)

curl 'http://ez.loc/api/ezp/v2/content/types/60'  -H 'Accept: application/vnd.ez.api.ContentType+json'

both have "isISBN13" set to true in the field settings.
Note: in legacy while editing a content of this type or viewing the content type, the type of ISBN is correct.