Is it normal for Lichess.org rating to drop after a win and why does this happen?

This can happen on Lichess because Lichess uses Glicko-2, not classic Elo. Your displayed rating includes rating deviation (RD), which adjusts as the system becomes more confident about your strength.

After a win, if your opponent was much lower rated or your RD shrinks significantly, your rating estimate can move down slightly even though the result was positive.

This effect is more common in rapid/blitz and after periods of inactivity, and it stabilizes as you play more games.

If you have experienced a similar issue, please feel free to share. You can also file a complaint on Xolvie.