I had a somewhat similar experience as yours. I had commented on a post and included a link, for me the link worked fine, but there was one user that it was sending them to another post. Here is the link that I had posted https://lemdro.id/post/4319, which is a link to a list of Lemmy apps. However, the other user was being sent to this post, https://lemmy.ca/post/4319, which is a post that says “Tesla now accepts bitcoins”. Same post number, just different instance. The user that was being sent to the wrong post, is on Lemmy.ca and said that they use Connect for Lemmy. Here is a link to the comment thread that I am referring to: https://lemmy.world/comment/1129638. I am on Lemmy.world, and the post opened correctly for me using Connect, so i didn’t really know if the problem was with Connect for Lemmy, or with the Lemmy.ca instance, or maybe a combination of both.
I know this all sounds confusing, but I thought I would share my experience, in case this could help the dev possibly figure out the problem.