If a failure occurs with receipt signing on the part of fiskaly or FinanzOnline, you'll notice this both through a notice on the receipts themselves and in the top left of the app.
In this case, the receipts need to be recorded retroactively. To do this, click on Record now in the notice message and then on Start retroactive recording. The retroactive recording now runs automatically; once it's finished, click on Done. No further steps are necessary.