Wouldn’t this be exactly the opposite case? In my scenario Cloudflare directly receives the mail from the sender (without any forwarding in between) and reports that the DKIM check failed, but at GMail (to which the email is forwarded by Cloudflare) the DKIM check succeeds.
In the thread that you linked it seems that the DMARC fails after the forwarding, indicating that the forwarding might have broken either DKIM or SPF.
The difference I see is dkim fail vs neutral. Certainly very different statuses. What I’m wondering is why did dmarc fail in that other thread. And why are there inconsistent results from Gmail? You’re right, though, they’re backwards. They fail in opposite directions.