That's because wallets post their own pubkey to the event and not the pubkey of the user that made the zap (that's held in a property on the primary event). NostrGram doesn't currently break-out and index the inner Zap event so has no way to know that you sent one to that message until it opens it up. If that makes any sense at all lol
Discussion
Ok thanks what's strange is that the "number of sats" zapped is added up and is displayed correctly (for example I can see that I tipped you 21 sats on this notes) but the little bolt sign stays empty.
So it's just a matter of knowing if I already tipped a note or not to sum up, like more a "design" thing I'd say
Yes the sats figure is aggregated on the server side the moment the zap hits, which is how it knows the value sent to any given note. I need to add an additional indexed field for who sent the zap as well. Just haven't done it yet (it's a big change on a database of 20 million notes!)