A new address is created automatically every time you transact for security and privacy reasons (with the exception of Ethereum and Ethereum-based coins and tokens). However, all your previous addresses still work fine. It is perfectly safe to use your Coinomi address to receive mining payouts or any kind of transfer.
You can see a list of all previous receiving address by clicking on "PREVIOUS ADDRESSES" link from the "receive" tab of your wallet.
All previously used (and unused) addresses are still part of your wallet and any coins sent to them will count toward your balance. Addresses never expire. While you can keep using them indefinitely, address reuse is strongly discouraged. Read more here.
Note: If you receive mining payouts in Coinomi and receive errors when sending the coins out, you are most likely spending too many small inputs. This makes the transaction size (in bytes) too large to be accepted by the network. Try sending smaller amounts, or consolidate your payments into a single address occasionally. See also: I get a Broadcast Error when trying to send or exchange. If your mining pool allows it, it is recommended that you choose to receive less frequent but larger payouts.