When any sourcelink is placed within 5 blocks of an ME Interface (same range as source jar) it sends spare source to the interface once any surrounding source jars or source drives are full, making compact systems like mine end up like this once full of source:

Is this intended functionality?

Ars En. V1.1.2 btw

Assuming you've already been using an interface to receive source into the network, this is how it should work. I'm afraid I don't know of any way in which it might be prevented from filling up further.

Now that I remember my own codebase a bit more again, it turns out this is a pretty easy thing to switch off. Not sure when I'll get around to releasing a new build for ArsEng, but expect that next build to no longer have sourcelinks provide source to interfaces or pattern providers.