I believe 95% of this is due to the fact that nostr zaps and lightning are separate. Meaning separate protocols.

Your Damus told nostr, I zapped this npub. The npub had not yet confirmed receipt. And may never do really, which will timeout after a bit. Regardless if the sats were actually delivered or not.

Sometimes this happens due to silly things like maxing NWC budget. Unsuccessful lightning payment, which happens every now and then. And lastly, receivers client not being zap native, or did not confirm receipt.