The answer is yes. If the SYN/ACK reply to a SYN packet to port 80 comes back from port 8080 (instead of from the expected port 80), then TCP will drop the reply packet. The reason is that if the source port in the SYN/ACK does not match the original destination port of the SYN, then as far as TCP is concerned, the incoming SYN/ACK packet is not related to the original SYN packet it sent out.
When a client's connection to a server is DNAT-ed from port 80 to 8080 by a router, all replies to that client will have the reverse transformation done to them by the router. That is, the source port on all replies will be transformed from 8080 to 80. Thus, to the client, it looks like the server is actually listening on port 80.