It's like @JmanTheBoss said. Weapon upgrade behaviour was changed in 'last' patch.
I tested it myself with the crossbow. After upgrading weapon's EXP started to count again and I'm at MAX weapon level now. Although I think displaying 0XP is misleading, it would be better if game would display 'UPGRADE REQUIRED' instead of 0XP.
You will now only gain XP toward weapon upgrades if you have purchased the prior upgrade for that weapon, i.e. level two must be purchased to be able to gain XP toward level 3.