Please may I help with a little information?
Context is everything! I remember the time very well. By 1987, it was clear that the marriage of Princess Anne and Mark Phillips was in deep trouble. They rarely appeared in public together and were often linked with other 'romantic partners'. Indeed, as was later discovered, by that time Mark Phillips had in fact already fathered a daughter with another woman in August 1985...
Up until the time that Anne was created Princess Royal it was customary to refer to the Princess as 'The Princess Anne, Mrs Mark Phillips'. Obviously behind the scenes it was obvious that the marriage was going to end in divorce. [Do remember that those days that back then 'royal' marriages did not end in divorce, [it was almost unthinkable] certainly not in what would be termed the 'respectable' marriages - Princess Margaret had divorced, but she was always regarded as somewhat 'flighty' when it came to romance [ Peter Townsend, Roddy Llewellyn, even her husband Tony Armstrong-Jones]. Behind the scenes, the marriage of Charles and Diana was in deep trouble, but generally speaking divorce was hardly countenanced in royal circles - look at the Earl of Harewood. He had a colourful' personal life, with a divorce and a 'pre-marital child'... and post-divorce he was never 'received at Court' again.....
Once Anne became Princess Royal' even though she was still married to Mark Phillips at the time, she was thereafter referred to as 'The Princess Royal', with no more reference to her being 'Mrs Mark Phillips'. By 'breaking the link' between Anne and Mark, the public were in effect being 'softened up' for the separation and subsequent divorce of the couple....