Vanilla, all DLC except Penumbra and all add-ons except the Pug.

Best visualized via this video:  

Pertinent ship loadouts and hero upgrades on the fleet in question:

I think it was working fine and then I traveled to another system and it went back to almost no range again.

This autosave is a couple of turns before that issue so you can recreate it.  Happens on all my fleets but haven't narrowed down why.

AutoSave 23924.sav

Some more evidence.  Looks like the vision dies when you enter an enemy system, at least in this example.  Hopefully this is ample info for you guys to recreate and fix.  I don't consider this to be minor.  Fleet vision is key to preparing for what's coming, and it's easy to not notice that it isn't functioning correctly.