The answer is neither, I think. It didn't expire because it was interrupted before the date of expiration. It wasn't extended either, a new 15 year period began.
The new (extended) period of investigation is a 7.5yr period, not 15, commencing from the date of the interruption.
https://curia.europa.eu/jcms/upload/docs/application/pdf/2019-12/ndr-2017-005_synthese_en_neutralisee_finale.pdf54. In Portugal, public prosecution is time-barred where, from the starting point of the
limitation period and excluding the period of interruption,
one-and-a-half times the
duration of the standard limitation period has elapsed. Where, under specific laws,
the limitation period is less than 2 years, the maximum length of the limitation is
twice that period. According to case-law, the purpose of that temporal limitation is to
prevent offences becoming de facto time-barred as a result of the successive
application of multiple grounds for interruption and suspension, and thus to stop the
interruption of the limitation period being “dragged on interminably”, which would
be contrary to the foundations of the institution of limitation.