Didn't there used to be an option to unload instruments when archived? I don't see such an option now, but would have sworn it existed in 8.5.
 
At any rate, they appear to NOT be unloading currently, at least according to Task Manager. When I un-archive them they re-activate immediately, which also suggests they are not being removed from memory. This is not relevant to the OP, just an observation.

Unless I'm missing something obvious here, Bitflipper, I think your comment is very relevant... In this project I have frooze and archived 2 instruments when playback got buggy. Seemed to work as expected but trouble started again as soon as I loaded up ONE new plugin. Problem might be caused by my many instances of Waves GTR, but then again those are frozen (with fx bin option checked)