If:true is not working properly in LWC, Can anyone please explain?

The reason why if:true={value} equals false is because 0 value is falsy. if:true directive expects boolean context, thus making type coercion. I'd recommend you to use getters. In your case, it can be looked like (if you expect the value property of number type):

@track value = 0; get hasValue() { return typeof value === 'number' && isFinite(value); }

Hope this wil resolve your issue.