Various solutions for data transmission enhancements with respect to user equipment (UE) and network apparatus in mobile communications are described. As a transmitter, a UE may determine whether specific information is comprised in a protocol data unit (PDU). The UE may mark the PDU in an event that the specific information is comprised in the PDU. The UE may further prioritize the marked PDU and deliver the marked PDU to a next protocol layer. As a receiver, a UE may receive a marked PDU comprising the specific information. The UE may deliver the marked PDU to a higher protocol layer out-of-order.