devtrackers.gg
Thanks for pointing this out again - this is something we would like to see improved as well.
TLDR - There are a few things we can try to make this better, making sure this gets tracked for us to look at again internally.
As for the more detailed how and why - technically there is no structural support for the building grid in that spot because it is made up of static geometry that was built with specific dimensions to fit behind building pieces (I.e. you still need to be able to build walls flat against this piece without clipping), not support it, and therefore doesn't extend far enough to register a connection based on collision overlapping into the neighboring grid tile.
The railing is a decorative prop and does not count as a wall piece.
For optimization reasons we avoid filling spaces like this with additional wall pieces that could otherwise count as supports.