When I was at the Se in Lisboa to collect my sello (had a credential in advance) in 2018, the office was not open at 10:30 a.m. so I just started hiking and collected my first stamp at a little cafe on the north end of old town. The Se office had hours posted on the door but those appear to be suggestions?!!
In my experience, both in Spain and Portugal, the timing of openings and closings are sort of flexible. No worries, just keep hiking.
Good luck and Bom Caminho !