Once you reach level 50 in Horizon Forbidden West, any player XP rewards will no longer fill the XP bar. This is because you have reached the level cap and can no longer level Aloy up. For more information about earning XP, see our guide: Horizon Forbidden West: How to Get XP and Level Up Fast.
However, you will still accrue Skill Points by completing any outstanding Quests, so you can continue to grow by investing in the game's skill trees. For more information about Quests, see our guide: Horizon Forbidden West: All Quests.