I think it may just be Reddit being inferior software, but it happened to me, too. I have been Googling “Reddit “ and my username, deleting what comes up, and doing the search again in a couple of days to see if it stuck. (Deleted comments will show up on Google for a few days because of caching, so there’s no point in checking immediately). It takes a while, but it’s fairly reliable, and it can be done in small batches.