Also encountered it.
I could easily live with that if it was just build order.
But the fact that it reverts back to being slow after belt backs up once, makes this tech (sideloading a splitter) kinda useless :cry:
It's not that hard to get miner to output >0.5 of stacked belt, especially on Fulgora

When manually upgrading belts using ghosts, game is trying to place undergrounds if you started dragging from already upgraded section.
Video demonstration is attached \/