Steps to reproduce

• Find/grow a nice long vine
• Place the gravel block against it
• It will cause a suspended vine (but that is a separate already reported bug), and it will cause the gravel/sand to be suspended
• If you look on the bottom side you will notice a vine holding it up, therefore the bug is that vines on the underside of a gravel/sand block will hold it up, but not one which is against a block (you can reproduce this by dropping gravel/sand a block or more above a vine which is placed against a block)