Either there is a problem with the buffing, ot with the timing of the system.
I just triggered premium.
I waited until the pop-up message acknowledged that premium had started
I then went over to a friends island and buffed a few buildings (with badges)
He asked if I forgot to trigger the premium.
I went back to his island to check, and the time remaining for the buffs was 8h58m - the extra 10% premium time had not bee added to the buff.
Either there is a problem now due to the display of the time (probably not) or the database did not update itself correctly in the short time between the acknowledgement and me applying the buffs. Maybe there is nothing to do except wait a little longer, but...