Ok I tested with the latest build and it did not resolve the memory leak. I tested on both debian buster and on Arch clients, they behaved similarly. Some things I noticed: the memory increase typically occurs alongside mediacenter pegging one CPU core for somewhere between five and ten seconds. It also seems to coincide with SSDP broadcasts, but that's harder to say as those occure pretty frequently anyway. For a test, I tried disabling all non-jriver SSDP devices on the network, but that didn't change anything.
I'm stumped!