Google used to do this for low volume emails I was literally sending to myself by authenticating with my google smtp account credentials. I could find no rhyme or reason to it. It would claim to accept emails and they would never show up in my inbox or in spam. Sometimes they would get delivered to spam. Sometimes to my inbox like expected.

It most often involved security log snippets which would include ip addresses and hostnames that were likely already on "is a known bad actor" list.

Bug on their end? Intentional thing to disrupt spammers? I don't know.