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Intended.  Or, more specifically, a special case that we didn't intercept.

Reactions are not "baked in"; they're defined by each community.  We seed some default ones, but a community can remove them, add others, ore even edit them.  *Sometimes* it makes sense for you to be able to react to your own post; for example, you might not have time to update an answer right now but you want to mark it as outdated in the meantime.  We didn't give "works for me" any special status.

If we wanted to prevent (some) self-reactions, I think the correct way to do it would be to add a checkbox when creating/editing a reaction to indicate whether self-reactions are permitted for that reaction.  That's possible but adds a little more complexity.  Is it worth it?  Reactions are signed (by design), so self-reactions are obvious.  If a community considers them poor form, the community can probably discourage them in the same ways it encourages or discourages other behaviors.