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.
How would you even know? Last I checked, I was unable to find any support path to Gmail where one could report email-blackholing-instances and get them investigated.
This is what I wrote several years ago at the point when I gave up trying to run my own email server:
> Over the course of 2 years that I was actively using my own server to send email from the same domain and same IP, there were periods of time when my emails were landing in Gmail’s inbox. And then there were periods of time when my emails were placed in spam, sometimes outright bounced, and sometimes received and dropped without even landing in the spam folder. You could never know what Gmail would do with your email.
https://www.attejuvonen.fi/dont-send-email-from-your-own-ser...
However, how do I as a user report that an email that was accepted by Google's own SMTP servers was not subsequently delivered to my Gmail inbox? I can't call or email support and it is a meme at this point that Google support is only accessible via social media or contacting a Google insider.