Summary:
Blocks like carpets, big dripleaf, all plants and other blocks that require a supporting block are not visible on the map if they are placed on the air or on incomplete blocks like farmland, big dripleaf, carpets and so on.
The exception is the bell and signs. They display correctly in any placement.
Because of this, plants cannot be seen on the map (although they can be displayed if placed on the grass with a command).
For comparison with java version: in java version, incomplete blocks can be displayed on the map in the air.

Steps to reproduce:

1. Place any incomplete block.
2. Place any block on it that requires a supporting block.
3. Create a map to see what a block that requires a supporting block looks like on the map.

Steps to reproduce 2:

1. Place in the air with the command any block that requires a support block.
   or
   Place blocks that require vertical block support (so that there is no full block under them).
   

Observed results:
Blocks that require a supporting block do not appear on the map if placed in the air or on an incomplete block.

Expected results:
Blocks requiring a supporting block will always be displayed on the map.

Screenshots:
Bedrock:

Java: