They use a plugin to mimic 1.13 and 1.14 joinability/compatibility, so the player base can remain more open. This plugin does an alright job, but it wasn't coded to show all block states/blocks properly, so you get many visual bugs. It's highly recommended to play in 1.12.2, which the server is actually based, to have the most bug-free experience.
You are on 1.14, This happens in 1.13 too, Try going on 1.12.2 its recommended so fences, doors, etc can be fixed. 1.14/1.13 isn't right yet.