If you place spruce leaves where a spruce tree would grow new leaves, those existing leaves will have their persistence state set to false and will decay along with the generated leaf blocks if decay conditions are met (logs removed).

To reproduce

1. Place some leaves in a column
2. /gamerule randomTickSpeed 300
3. Place a sapling next to the leaf column
4. Wait for the sapling to grow into a tree
5. Break the tree's logs

Observed behaviour

The previously placed leaves disappear.

Expected behaviour

The previously placed leaves stay as is.