In 1.12.2 mushrooms popped off when a block was placed next to them while it was too bright. In the latest versions they only pop off when a block is broken next to them, but not when it is placed.
- Build a dark room
- Place a mushroom inside
- Place a sea lantern 2 blocks away from the mushroom
- Place a block on the other side of the mushroom
- Observe the mushroom not pop off