I'm a British citizen (by birth), born abroad in the late 1990s. I recently found my first passport, and in it are two UK entry stamps from my first two visits to the UK. One is from Gatwick and the other Manchester, so this evidently wasn't a one-off or a mistake.
I was a baby at the time, and my parents don't remember anything unusual happening at the border. I haven't had any UK stamps since, and I can't find anything about this online.
The stamps were unremarkable and don't specify any limited leave to enter, except one has letters that look like "O/R" handwritten underneath it (as pictured).
Why would the border officer stamp a British passport? Photo of one of the entry stamps