With Whirlisprig Charm in the hand, right clicking any flower block (even Flowering Azalea Leaves) will create the whirlisprig flower without any checks to see if the below block has a solid face. This can lead to odd looking floating whirlisprig flower blocks.

You could override canSurvive method to return true only if the blow below is valid and have the charm check that first before converting the block. It will also make the flower autobreak if the player mines the block from under the flower, thus helping to reduce cases where it can be left floating in the air.