Light seems to be working correctly for me: the weapon affected has a nimbus of light and the nearby environment is noticeably brighter when it’s equipped than when it is not. I tested with Gale (who is the only character in my party with the cantrip) holding both a staff and a crossbow. So at least this doesn’t seem to be a universal bug.
Unfortunately, since Patch 9, it's not necessarily the weapon currently displayed on the character (which, is an issue indeed). I had to click on the weapon instead of the character (and that's a bit tricky).
I’ve not noticed the spell not affecting the equipped weapon, so I’ll look out for that from now on. But out of interest, how do you target the weapon rather than the character? Do you mean clicking on the weapon graphic displayed on the character sprite, rather than elsewhere on their body? I can’t see a way to target the weapon on the hotbar or in the inventory.
EDIT: I should have said, Light is *displaying* correctly for me. I’m not sure whether or not it’s having the correct gameplay effects.