Derp... after a bit more searching, I discovered this is a known issue. I've found a bug with the thrust plate part in symmetry and parts scaled by Tweak Scale attached. It appears that, in symmetry, parts connected to the thrust plate are attached as if the haven't been scaled at all. I only had Engineer Redux, Editor Extensions, Part Angle Display, Tweak Scale, Procedural Fairings, Module Manager, and some re-textures of stock parts installed when I created the following images. To reproduce the bug, I placed four LV-T30 engines, scaled to 0.625m, on a 1.25m thrust plate. They were attached individually as to avoid any weird symmetry bugs. Then I added 4x symmetry to the radial decoupler. The same thing happens if the engines are upscaled aswell. And as a sanity check to just see if it might be a bug with Tweak Scale, here a few pictures with the plates removed and stock parts used. Same method as above: add decoupler and fuel tank -> add stock adapter, if needed -> add scaled engine -> add symmetry on decoupler. Scaled up LV-T30s: Scaled down Mainsails on quad-adapter: Scaled down Mainsails on radial adapters: