Light blocks can be replaced by placing other blocks in the same space as them

Light blocks are replacable, just like grass and structure voids; this is because it is invisible and has no collision. It's even included in the #replaceable block tag (which doesn't control the behavior, but does list every replacable block).

Not a bug.

Updated this report's title to be clearer as the potential issue here is that light blocks are replaceable. While I'm pretty confident that this is intentional, it's ultimately Mojang's decision.