Various solutions for supporting out-of-order hybrid automatic repeat request (HARQ) feedback with respect to user equipment and network apparatus in mobile communications are described. An apparatus may construct a first HARQ-ACK codebook corresponding to a first physical downlink shared channel (PDSCH) and construct a second HARQ-ACK codebook corresponding to a second PDSCH received later than the first PDSCH. The apparatus may activating an out-of-order HARQ transmission to transmit the first HARQ-ACK codebook and the second HARQ-ACK codebook in an event that the priorities associated with the codebooks are different.