Hello @Xaymar, are you still experiencing this issue on an up to date system?
It seems not, since the server has been running stable for over a month now with no sign of running out of memory or leftover processes. I have to admit I entirely forgot about this at all until now. :/
Closing as resolved.
Nothing extracted yet.
So for some reason, the binaries that steam updates to cause a memory leak and a leftover steam instance that takes up an entire CPU core.
I've observed this both in automatic commandline/scripts mode and manual command mode, where it would output one last "Steam>" line and then move into the background by itself and waste resources there.