The odd thing here is, the times I'm losing AF3 and not gaining UI3, I'm seeing my damage value pop up. There are occasions where I don't see a damage value pop up like you're describing, but I either get the full switch to Umbral Ice III (in the case of a mob dying as my spell lands) or I stay in AF3 (in the case of a mob being invulnerable). I've attributed the lack of a damage value popping up on occasion to a graphical oddity (though this has only really been an issue since 2.3 as well) because it's not limited to when a mob dies/goes invuln (in which case I usually get the "invulnerable message," though not 100% of the time, though I do always get the sound effect).