I've downloaded the previous version Cyberduck 220.127.116.11621 and can confirm that it WORKS there. Hence this must have been introduced with 7.3.1.
I'm quite sure that this memory fix does not work properly. With the old version the RAM memory usage stays < 500 MB during the whole download and does not increase permanently. It seems that every MB that Cyberduck is downloading with 7.3.1 is added to the RAM usage.
Small update: The RAM usage DOES increase with CyberDuck 18.104.22.168621 but not close as much as with 7.3.1. After downloading 17.5 GB from the online-only vault (OneDrive) CyberDuck 22.214.171.124621 consumes about 2,5 GB of RAM.