That would explain it, but also, that's super broken

Nothing broken about it. It's optimized for a particular situation, that situation being a long running process on a server. This is where the JVM typically runs. If you don't want that behaviour there are a myriad of GC options, which could be better documented but are not that hard to find.

I'm not the one who wrote it though, and it's software designed to run on a workstation.

It's not a big issue for a server deployment where if you got that memory from the OS and didn't get killed, there's probably nothing else running on the box and you might as well keep it for the next traffic spike. But yeah not ideal on the desktop/workstation.