Buggy Scaffolding Placement

Do you have the same problem with a vanilla server?

I do recommend you test on a fully vanilla setup. I am wondering though, what exactly is the bug here? As far as I can tell from the video, it's working normally.

I experienced the same problem in my vanilla creative world. I am unable to place scaffolding up when above it.

Are you right clicking on the side of the block? It may take a while to learn how the placing works, but I'm am pretty sure the behaviour you are experiencing is intended.

If you want to build upwards while standing on the existing scaffold, press the sneak key while placing on top.