If you've found a bug in
rtmpdump, posting it on the rtmpdump mailing list is the way to make sure it gets attention.
Fwiw, it looks to me like rtmpdump's resume is actually working fine, it's just printing the wrong percentage for its statistics.
E.g., let's say you got 80% of a file and then had to resume. It should say that it's resuming at 80% but it will say it's resuming at 0% instead. It will get to the end and report that only 20% was downloaded, because it's not taking the previous 80% into account in the stats. But actually the file is complete...