I just had a similar situation happen to me in a ranked game playing Yorick top.
- Jungler initiates bad dive attempt on enemy Rumble, we both die but my ult rezzes me and I chase Rumble down for the kill.
- But I didn't actually die! Wow, that went better than expected I guess then. I thought, perhaps, I only came close to dying and my ult just expired.
- A few minutes later, me and Rumble are back to regular laning and I get three-man ganked. I try to ult myself and fight because I think I can kill Rumble. But I can't ult myself.
- As my HP runs out, I immediately appear back in spawn at full HP but opponents don't get kill credit or gold for my death. Everyone is all "WTF just happened?"
So presumably what happened earlier is, I did actually die during the dive on Rumble and my ult rezzed me. Rumble even said he wondered how he got the double kill since I seemingly didn't die. I did die, the ult just never actually expired. So I continued to "live" through my ult and then when I got ganked a few minutes later, they killed my "ghost." That would explain why they didn't get a kill + gold, because the game thinks I'm already dead, and also why I couldn't ult myself, because the game thinks it's currently active.
Certainly not an ARAM-exclusive issue. Also, my ult worked correctly the rest of the game so I have no idea what might've even caused the problem that one time or how to reproduce the bug.