Resolved:
Reddit itself doesn't filter. Reddit HFY does.
It is not possible to link from Reddit HFY to Lemmy HFY directly due to shadow-filtering. The whole message with the link will be only visible for the creator. Working around with eg https://lemmy(dot)world/post/94994 though works as long as the user edits the link while copying it.
old text
Does Reddit block Links to Lemmy.World?
I just wanted to post a link in a Reddit Answer. To my utter surprise the message was not shown to other people. I as the author could see it without any hint it would be invisible.
But everyone else couldn't see it. It just wasn't there, even an direct linkt didn't help. Editing the message didn't help either. I had to delete it and rewrite it without mentioning "lemmy.world" in the message.
Here is a workaround where I avoid a link and it works:
Reddit Post without Link Workaround
Edit 1: To make myself clear again, you should see at least one message without a link and one with a link to Lemmy. But you will only see the one without a link. The trigger seems to be the exact word “lemmy.world”. Is that a known problem?
Edit 2: Wow, this goes deeper. A fresh Edge browser does show the link post. A old Edge browser doesn’t. All browsers with strict privacy settings don’t show the link. Firefox in Private Mode and with Adblock doesn’t show it. Edge and Chrome in private mode don’t show it either. A fresh chrome doesn’t show it. An older Installation of Chrome does. Reddit is doing something very strange.
Edit 3: https://lemmy(dot)world/post/3375662 seems to bypass the filter. So it is true: Reddit filters their biggest competitor.
Edit 4: The Shadow-Ban doesn't seem to work on r/help but definitely on r/hfy.
To give some more information on this, reddit can "shadowban" users, which will cause all of the user's submissions and comments following the shadowban to be automatically removed. Moderators of subreddits can see and can "Approve" these removed comments on subreddits they moderate, but only reddit admins can remove the shadowban from the user if they appeal. This shadowban system is often used on spammers so that they waste time spamming into the ether without realising it, however the shadowban would affect all of their comments and submissions, not ones with a specific link in them.
Reddit does also maintain a list of domains commonly used for spam purposes. I believe ibb.co is an example of a domain on that list that instantly gets any comment or submission it is in removed.
When a comment is removed, it is replaced with "[removed]" text, however this can only be seen by other users if there are any replies to the comment. Otherwise the comment disappears from view except to the user themselves and the subreddit's moderators.
@Crass_Spektakel you should try testing this in places other than the subreddit you have seen this in. Subreddit moderators can apply their own link filters and automatically remove comments and submissions without notifying the user.
I updated the original post, it is not Reddit but Reddit HFY.
Gawd, the way Reddit tries to shadow ban is like a five minute hack for me to counter with a tiny perl script. It helps absolutely nothing against bots but annoys normal users.
Thank you for doing that :D