After some discussion, we have come to the conclusion that disconnecting from the game during a run, in order to pass keys to another player (which are always received by the game's host), should be allowed as a valid strategy. Reconnecting once you have left, however, will result in your run being invalidated.

This may not be extremely impactful in a lot of cases, but if people can use this mechanic to their advantage, I think it's fair to allow it.