That charger is only a 4.9j charger. I have no idea what the brush burden is on your fence but I have a 24 j charger for about 3x that distance. It should have a low voltage shut off function in the manual and I suspect that’s the issue but I don’t know. I have a 12j charger for the low wire since that one is the highest burden. In my experience most people don’t realize how many joules you need to push on a line with lots of small losses. Once you have a larger charger then you can trace the faults.
To give you an idea, my 24j charger will normally put out ~8j to charge to 8kv, but a few branches on the wire will make it push double the joules for 5kv, but that allows me to trace faults. I’ve also found that my buried insulated wires (Gates and initial wire run to the fence) are very significant losses. Like 1A per 20’. I have a few trees as posts and even with insulators I get induced current losses (pushing high voltage will do that) to where I can measure current in the tree!
Thought experiment- overheard power lines are 12.5kv and look at the insulation used. Little dinky tube insulators of crappy plastic with moisture will make losses pretty high.