I am a dual EU/UK citizen (recently naturalized in the UK). Since the end of the Brexit transition period, I have tried to collect stamps in my UK passport. I travel frequently between the EU and the UK, and sometimes ask the European immigration officers to stamp my British passport. They ask if it is a souvenir, to which I say yes, and very kindly and smiling they stamp it. In two cases though (Switzerland and Italy) I didn't get an entry stamp, either because I was using the automatic gates with my EU passport or because the line behind me was too long and I didn't want to hold up the people behind. I did get an exit stamp when leaving those countries though.
My question is: Would a Schengen exit stamp in my UK passport without the corresponding entry stamp cause a problem if I travel to (say) Brazil, Mexico, South Africa, or Canada? Or would foreign immigration officials just not care about Schengen entry/exit stamp?
(I always carry my other passport with me, so I'd be able to prove that I am an EU citizen and don't need a stamp anyway, and those I have have been affixed on request.)