Perhaps they brew for a while before their first season - my first was a Lab bitch but that was over 20 years ago and I can't remember her going through any sort of personality changes - she was so laid-back she probably didn't !!
We had Bramble neutered at about a year old, but it took a further 6 months before his 'silly hat' came off....
Both he and Barley have reacted strangely (as far as the human eye can tell) to various people from time to time, as well as taking a dislike to people with pushchairs, umbrellas, gloves, all of which they have seen before, and none of which are at all sinister taken in isolation ...
It is really embarrassing when they suddenly back off and bark at a completely innocent person, but I just excuse them by saying 'Who knows what goes on in a Spaniel's head??!!'