Knowing the APP assigns the address
Wallets do not assign recipient addresses for you.
Setting the fee and the miner address to send fee to
You do not select a miner address to send a fee to. The fee in a bitcoin transaction is simply the difference between the inputs and the outputs. Any output address you see in a transaction is an address belonging to someone else, not to a miner.
It sounds like TheArchaeologist is correct here - your wallet has been compromised, your deposits are being stolen by a third party, and you are mistakenly believing that they are being sent to a miner to cover the fees.