It could be:

1. Recipient's LN node doesn't issue a Zap receipt event

2. OR it is issued but it gets lost/deleted on the relay (less likely)

3. OR Yakihonne checking for the Zap receipt event incorrectly

I would say most likely the problem is 3.

And even if it's not Yakihonne, I would definitely apply a timeout on fetching the Zap receipt, like 10 seconds, then I would just alert the user that the payment might have failed