The written rules seem to state that the only requirement for funding another player is having at least 20 more Np than the other player.

A player just lost a mine and seemingly stopped funding another player around the same time. I'm not sure whether the two are related. Is there an unwritten rule that not having a mine prevents one from funding even if one has more than the required Np?

losing a mine makes you lose 20% of your neptunium. that could have dropped that player below the 20 Np lead threshold.

Nope, the funder currently has the required Np according to the intelligence report.

Then either they stopped the funding themselves or they temporarily dropped below the 20 point lead and it got cancelled automatically.

This may have been our current game Czech?

The funding was canceled on me for diplomatic reasons.