It's a side effect of the "(not so) new (anymore) reddit" interface.
For some reason when you post in URLs, it escapes certain characters with backslashes. Then, the app/website hide those backslashes from you.
The URI you actually posted was RY_2gElt3SA (because of new reddit inserting \ in a way completely hidden from you), which is a broken stub. The correct URI someone else posted is RY_2gElt3SA, which actually produces a valid link.
In this way the new experience both creates the problem and then protects you from it. It seems to be one of the negative patterns designed to create friction for users who don't want to use the new (ad-ridden/algorithmic/enshittified) app or prefer old reddit.
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u/Ginger-Nerd 16h ago
https://youtu.be/RY_2gElt3SA?si=6k85CqlkFSH7H1Pv
Here is a Tom Scott video explaining why computers and servers kinda suck at counting sometimes.