When cutting into the earth to build a path / road the blocks will usually pop up when you fill in the spot with stone etc. Although when you place stairs, when there are still uncollected blocks beneath, the loose blocks will not pop up and remain trapped under the stairs. If there is no other place for them to move to. They should pop up, but they remain trapped under the stairs.

To reproduce:
1. Cut into the earth, place a stone block
2. Cut more dirt making a path
3. Place stairs into an open space that has uncollected blocks in the bottom
4. Blocks will remain trapped, but can be removed when the stairs or a nearby stone block is removed.