As you can see on screenshot, an hostile savage tribe (The Blue Mire Kinship) that I made neutral during negotiations had it's goodwill decreased by the natural tendency anyway.
(https://i.imgur.com/JRu6c9N.jpg)
This is not a bug, in my opinion.
Your faction is temporarily neutral with them, but because of the nature of it's culture your relationship will decrease if you do not put attention on them (trading or giving them presents)
I was also going to report this bug. It seems to me that if you garner enough goodwill to make them now "Neutral" they should only decay to the lowest Neutral goodwill level.
Quote from: Turtuhl on August 25, 2018, 02:43:40 PM
I was also going to report this bug. It seems to me that if you garner enough goodwill to make them now "Neutral" they should only decay to the lowest Neutral goodwill level.
That does not line up with a faction who's natural tendency is savagery or aggression.
I agree with mecatxis, I don't think this is a bug. I think that savage tribes should continue to naturally become hostile to you if you are not constantly maintaining relations.
While I see your point this game has always been about risk-reward. If you risked your colonists to gain enough favour to turn a faction the reward should be consummate. I will respectfully defer to Tynan in the final version.
It's as intended.