Apparently there are eGates at Lisbon where you don't need to get a stamp afterwards. Did you use one of those?
https://www.walesonline.co.uk/whats-...d-ups-24190471
There are e-gates at three Portuguese airports that UK Passport holders can use. They are separate from the EU e-gates used by EU passport holders. These are available at Lisbon, Faro or Funchal (Madeira) airports.
These will digitally record when a passenger entered and exited the country and there are calls for a similar system to be installed at busy airports in other parts of the Schengen zone. It is still possible to request a manual stamp as extra insurance.
EDIT: posted as the same time as the above, so this is no longer a viable explanation!