I have been leveling an APU recently and have experienced a bug I have heard others complain about as well.

When casting on a mob (haven't actually tested in PVP), damage numbers are shown, but don't seem to register.
Initially I thought this was lag, so I did some experimentation and ultimately concluded that this appears to be a bug which happens quite frequently.

I have found that this seems to be more common with beams than halos. I don't know if that is entirely accurate, but it appears that way.