Why is there an invisible solid object rising along with the floor beam?

Beats me. The best way to answer this kind of question is to go in the disassembly, dummy out the relevant bit of code and see what happens. First off, the solid background flag isn't set until the thing starts moving, so that sucks.

That would be easily fixable, though, so let's move on. If you let the beam carry you through the one-way platform, your character won't have any vertical momentum, so they'll get caught on the top-only block and fall behind the background scroll, eventually getting crushed by the sideways collision on the beam as it plows on ahead.

This could be fixed by removing the sideways collision on the beam chunks. Lastly though, and perhaps most damning, is the fact that the beam does not carry you horizontally as it moves diagonally up the screen.

This is likely why every other use of the solid background flag only has the background plane scroll vertically. Even the crushing wall at the start of Hydrocity Zone 2, which moves horizontally, does not let you stand on top of it.