now that's weird!
i'm sure the pickups are grounded, or they wouldn't work. (still, it's worth it to check, that the pickup braided leads and the backs of all the pots connect to each other, and to the sleeve of the output jack.) is there cavity shielding in the bass, like graphite paint? be sure that stuff is grounded.
finally, there's no reason you couldn't hook the bridge ground back up, that doesn't affect anything and would make your problem go away.
Guitar Repair Gnome
Alpha Music, VA Beach