After the 1.20 update, coral blocks (not fans) require support below them to be placed.

How to reproduce:

Attempt to place a coral block 1 block off of the ground or on top of partial blocks such as fences or glass panes

Expected result:

The coral block should be placed

Actual result: 

With several non-full blocks, or no block below the location where the coral block is attempted to be placed, the coral is not placed