The first thing you should note is that the link you showed indicates that the link does not have Discussion Forum Schema. That link has “Breadcrumbs” schema only, no “Discussion Forum” schema at all. That’s happening because you’re testing the link in “smartphone” mode, and not in “desktop” mode.
https://search.google.com/test/rich-results/result?id=TlLcA6saLMo3BrxbQYnFuw
When you switch the link to desktop testing, “Discussion Forum” schema appears, and it flags the “missing field url” issue.
- https://search.google.com/test/rich-results/result?id=uy3Ub3IwJiIuaMh-7YUu6g
- https://search.google.com/test/rich-results/result/r%2Fdiscussion-forum?id=uy3Ub3IwJiIuaMh-7YUu6g
To reproduce the critical errors, you have to test a long thread with the ?page=2 URL parameter, like this one:
