This is not a bugs.
You didn't understand it correctly. You got 1 VG after shield. That VG expried, and you got attack and get another VG.
Here is the timeline:
You got shield 12 hours 36m ago,
36m ago, your shield expried, and your VG kicks in.
6m ago, your VG ends
4m ago, someone attack you.
After the attack, you got a VG of 30m
I guess The VG start when they attack your village. 4m + 2m attack >>> 6m >>> You have 24m of VG left.




