I'm gonna make a guess here because I've encountered the same issue when building things on LN.

It sounds like a subtle issue with Lightning implementatios: internal payments have to be settled internally in a separate database and not in the lightning node itself (LND doesn't support self payments, afaik CLN now does).

When the second payment is made through the real LN, the invoice is picked up by the node but the Alby backend already settled the invoice.