Don't be alarmed if your XP bar disappears!It's perfectly normal - it means you have all the XP you can get. To finish it off, you do need to complete all the Help Wenja and Tribal Clash quests that reward a Skill Point. You can see which ones these are by hovering over them on the minimap. You will stop gaining XP once you only have these remaining so do not worry - your game is not bugged. You do not need to complete all of these orange quests - only the ones that reward a skill point.
To farm XP, you'll need to capture all the outposts/forts and complete every main quest, including the ones that aren't required for achievements. Once you've done that, you can reset all the forts/outposts by pressing
and going to Options then Gameplay.
You can increase the amount of XP you gain in two ways. First, you can go to the tribal altars. There are 12 and they show up on the minimap as blue totem collectibles. They give you a 2% multiplier for each one you find. Alternatively, every 10 villagers you gain in your village give you an additional 2% XP to your multiplier up to 300 villagers.
I needed to run through most of the outposts again without being detected (13/15). If you were stealthy the first time, this number would be less.
The game can glitch and leave you short a few skill points. If this occurs, you'll have to wait for a patch. Devs said they were working on it.