Me and a buddy of mine have been playing warptorio 2 with IR3 for a while and we have noticed that the rubber trees have almost disapeared, I have hugged coastlines for 10h playtime but have found none. After some consoltation with the Discord servers the prevailing theory seems to be that Rubber trees only spawn on worlds "that remind you of home" which from my understanding becomes rarer as the game progress.

The few rubber trees we did find were enough to help us progress so it has not impeded progress but is has been a constant annoyance that we can find more, especially since we are like two rubber trees away from just automating it.