I found a small exploit, if you place belts without having undergrounds in your inventory, it does work, and display the 'Missing underground belts', but it also gives you a free pair of undergrounds.

But apart from that, it works great.

Hi, I have not been able to reproduce this. What are you building over where you get refunded free undergrounds?

Nvm I found it, let me see what's going wrong

Fixed (in Factorio 2.0.11), it was refunding ghost undergrounds as real items. It also refunds properly in 2.0.12+, but the buggy underground placement means you sometimes get random gaps or marked for deconstruction.