I bought an Ibanez GSR200 at a local pawn shop for 70$, love its its amazingly light and thin with a twig for a neck(thats not a complaint mind you), after giving it a a proper set up it sounds amazing, however ive ran into a weird problem, when you lay the bass on its back or when you put force on the neck either backwards or forward, the strings raise and lower (like a built in whammy bar), its like the neck were made of rubber and you can bend it back and forth even under the tension of the strings and the truss rod, at first I thought it was loose neck bolts, but no they're on nice and tight, also its not the truss rod, as I straightened out the neck and a truss rod is fully functional, is this just a case of a really thin weak neck 

Also I don't usually lay any bass on its back, my basses spend 99.9 Percent of their non-playing time on their stands, I just happened to lay this one down on top of my bed and noticed the neck flexed under its own weight.