I came across today one of my own comments on Reddit with a search on duckduckgo. It was still intact. The problem is I used shreddit to randomise and delete the entire contents of my account. My account comments and posts all show empty on my account pages on reddit so there should be nothing.
I did a site:reddit.com search using my username and found ~50 comments that Reddit has undeleted but also hide from my own account. I could still edit and delete them. Its curious that they don't appear on my accounts content on Reddit and yet a search engine can find them and they are still served by Reddit. Ex Reddit users who deleted their account contents should be aware this is happening and report it as a GDPR breach to their respective agencies if they are in the EU if they too find this has occurred.
It's not you It's reddit profiles, they don't show more than the top N posts in whatever sorting order
You can delete everything in /user/new, then sort by "top" and notice there's still stuff there. There is no way to list it all and delete it all. And if you delete the whole account to at least anonymize the comments, they might bring stuff back (that you deleted) and there'll be no way to remove them anymore.