I've experienced something similar to this. I had a magma drainage tunnel emptying into an underground magma tube, sealed off from the caverns by a fortification at the end of the tunnel. Then, I placed a wall grate made out of basalt next to the fortification, out of paranoia that some flying magma-proof monster might find a way to swim through my fortification while magma was flowing through it.
Well, the first time I tried to empty magma through this tunnel, I looked at the wall grate afterward and saw that it was no longer installed, but just sitting there - as though it was stockpiled or just dropped there. At first I thought that I had somehow forgotten to actually build it, but then how did it even get there? It's not like there was actually a stockpile there.
I then loaded a previous save, from right after I had built the grate, and confirmed that it had in fact been built properly. So, it seems to be the case that flowing magma has some chance of dislodging built furniture made out of magma-proof materials. In this case, there were no mechanisms, it was just a static wall grate, so that couldn't have been the problem.
After the magma evaporated from the tunnel, I sent a dwarf in to rebuild the grate, and then flooded the tunnel with magma again. The grate remained upright from then onward. The one time that it deconstructed was the first and only time that anything like this has happened to me. But it definitely did happen.