21w41a, 21w42a, 21w43a, 21w44a, 1.18 Pre-release 1, 1.18 Pre-release 2, 1.18 Pre-release 5, 1.18 Pre-release 6, 1.18 Pre-release 8, 1.18 Release Candidate 1
Saplings can grow on invalid blocks, replacing them with dirt/rooted dirt, after being bone mealed. This includes air blocks, command blocks and bedrock. This, of course, cannot be done normally without commands, so I don't believe that it is a major issue or that it can be easily exploited.
This also does not seem to affect moss blocks, which are only replaced with rooted dirt by azaleas. Also, this does not affect Nether fungi, which do not accept the bone meal on invalid blocks. This also did not happen in 1.17.1. The aforementioned facts strongly suggest that this behavior is unintended.
Mushrooms do accept bone meal, but do not grow either.
1. Open a creative world.
3. Execute /setblock ~ ~ ~ spruce_sapling, for example;
4. Apply bone meal to it.
5. The sapling grows.
6. The block underneath it is replaced with dirt/rooted dirt.