Podzol does not turn into dirt when placed under a block

Podzol will not change to dirt, if it is under a block, even with a naturally generated one.

Steps to Reproduce:

1. Place a block of podzol
2. Place a block on top of it (e.g. dirt)
3. Wait for a while (or run /gamerule randomTickSpeed with a high number)

Observed Results:
The podzol never changes to dirt.

Expected Results:
The podzol changes to dirt, either immediately, or after a while.