Adding some… if the ammeter suddenly moves with that jumper wire added, the wire COULD be possibly missing contact AND in that moment the factory wiring entering into the game with an extra failure like loads incorrectly placed… or you are close to a short.
I’m just thinking out loud but that’s really weird and needs to take action before get into a bigger issue. The advantage of the ammeter over the voltmeter is it is a real and instant stage reading gauge. Also indicates what section of the wiring could be getting in troubles depending on which side of the gauges shows reading and also depending on how you wired it, because will tell you where is running the load. If reading discharge, the extra load is on alt side, if reading charge, the extra load is on batt side… this last just as far engine is running.
Extra load can be a temporal instant short not long enough to blow the fuse link. Althought with a jumper wire added, the fuse link is also somehow being bypassed.