No, it's not a bug. The funding fee for the short position on $BTC , like other cryptocurrencies, works like this:
1. When you have a short position, you are essentially "borrowing" BTC by selling them on the market. If the funding rate is positive, then you, as the short position holder, have to pay a fee for that position because you borrowed BTC from another trader who holds a long position.
2. When the funding rate is negative, the situation is the opposite: you get a fee because long positions "borrow" bitcoins from you, i.e. you get paid in this case.
So if the fee is always positive, it means that there is a lot of long (buying) activity in the market, and the funding rate is positive. In this case, you, as the short position holder, pay a fee.
This is normal practice on most cryptocurrency exchanges and is not a bug. It is worth considering that the funding rate may change depending on the market situation.
