I tried it out, both with my normal ui and with everything but rbm disabled - still the same problem. Arcane Shot, which shares a cooldown (and no one with ES ever uses it so I'm afraid I forgot about it before), also only shows its cooldown the first time.
There seems to have been another undocumented change to LnL, in that they once again removed the internal cooldown altogether. I can't see that really changing anything though. =/
Explosive Shot's cooldown timer is being very odd. If I reload my ui its cooldown will show up the first time I use it, but after that first time it never shows up again. All my other cooldowns are showing up correctly.