Losing Rank points discourages challenging yourself
Points earned towards Rank should only be negative when someone leaves an Outpost without completing it. You don't want to discourage people from attempting higher difficulty maps, but that's what the current system does. Persistence should be rewarded. As is, if you fail a few times, you might as well just cut your losses and leave without completing the Outpost. You're also better off sticking to maps that are quicker and easier to complete rather than attempting a map that may take multiple attempts. You also might want to consider not taking away rank points if a player dies in a base 10 times (maybe more, maybe less, what seems like a reasonable amount of expected persistence?) and then gives up. The person who designed the base got kills and the player attempting the base tried. Rewarding zero rank points is fine, but taking away rank points is unnecessary when the player made a real attempt at it.
(I know Rank doesn't necessarily matter, but it goes toward getting a reward at the end of the season, right? So it's a tool of encouragement. It should reward players for playing, and part of playing a game is challenging yourself rather than sticking to easy mode.)
Edit: Crossed out large sections because I changed my mind. Players shouldn't lose rank points at all. Rank is for showing time played. Rank is for rewarding cosmetics at the end of the season. Increase a player's rank quicker with better runs, sure, but don't subtract from it.