Factorio Forums

As per title - if you upgrade an underground belt with a single underground belt in your "hand" stack, only the nearby end upgrades, not the far end - even if you have more underground belts in your inventory

To recreate

1. Place a yellow or red underground belt
2. Attempt to upgrade it while holding a single underground belt, only the first belt upgrades

That's currently a limitation of how building works. We may do additional work in the future to try to find additional items for making both ends work but for now it's working as we programmed it.