As I could see, I don't think VIP Days is bugged...
Tradable VIP Days is used FIRST and after that the Not tradable VIP Days...
I just checked it out, let me explain:
- In January 1st 2020 you used a redeem code of 365 VIP days (1 year);
- You got a lot of extra VIP Days from quests, monsters, events... and at December 31 2020 you got a total of 1000 VIP Days (635 NOT Tradable and 365 TRADABLE);
- You did not traded any of the VIP Days in 2020, so you think: I got a lot of VIP Days to trade to trade in 2021;
- But January 1st 2021 arrived and the redeem code you used JUST EXPIRED cuz you redemeed it 365 days ago;
- All the 365 VIP Days will no longer be tradable and became NOT TRADABLE.
There is nothing wrong... you just missed the timing.