Yes, not sure what causes the over reaction on collision, will have to hunt though the GG code to figure that one out! But it isn't actually shooting off the map as you imagine, Lee put a 'fix' into the engine at some point which automatically destroys any objects with health of 0, the pickuppables script contains collision detection for thrown objects which forces their health to zero on collision (so that for example exploding entities will explode when thrown at something). So your thrown object is being destroyed by Lee's 'fix'.
To fix this you can comment out the line
SetEntityHealth( P.ObjectToEntity( thrownObj ), 0)
in the script but obviously thrown objects will no longer explode!
The line is in the function CheckThrowCollision() btw.
The jumping thing should also be fixed by that as it is due to the entity not being around anymore to reenable jumping!
Been there, done that, got all the T-Shirts!