Every animated Keeper and Preserver (non-heroic) appears to spawn with a fire halberd, never lightning.
Watchers appear to be heavily biased towards the fire or vitality halberd, very rarely acid.
Tested on Normal and Ultimate, in Cairan Docks, Korvan City, Temple of Osyr, Sanctuary of Horran.
So far lightning halberd spawns make up zero percent from Keepers/Preservers.
For Animated Watchers, the picture is less clear, very rarely I’m seeing a green halberd. Mostly red or orange.
Update: Maybe this affects the visual only, as I have just had a Keeper drop the lightning version, who looked like he had the orange version, at least as far as i could tell.
At 1%-3% drop rate this is pretty hard to verify, especially on a monster that spawns in such low numbers.
Update 2: After 100+ orange halberd spawns, I saw a my first regular lightning halberd dude.
I am guessing their weapon model is only replaced if they actually spawn with (and subsequently drop) the lightning/acid halberd, and the fire/vitality model is just the default (and the fire preserver dropping a lightning halberd might have been me being bllind). In that case it would not really be a bug and more a drop-rate issue, 1% for a monster that you see maybe 12-20 on a normal playthrough is rough.