In reply to neo alibe.

I analyzed some of the Google reports in more detail, and in some cases, it is indeed the itemprop to blame. My plugin always sets itemprop to item, not url, but it is not only that.

In few cases (same generator on the whole website), out of 60 pages, only 3 were flagged as invalid, and when the Google shows the markup of invalid breadcrumbs it highlights huge part of HTML outside of breadcrumbs, that has nothing to do with. I have similar pages, sharing the same structure HTML, and there is no difference, but Google flagged one, and not the other.

I think that some blame here is with Google also and some issues they have with the breadcrumbs on the Search Console. Because, even the URL’s they flagged, when tested in rich snippets test tool, they show breadcrumbs as 100% valid (no errors or warnings).