An answer for whether it is “normal” depends tremendously on:
1) what kind of harp
2) what kind of strings
3) what kind of environment (temperature and humidity)
4) what pitch of string
5) age of strings
6) is it always the same string?
For instance, if I had a different gut string in the first or second octave break on my pedal harp during rainy weather, it would be completely normal.
If I had a middle-octave flourocarbon on a light-tension lever harp break even once in ten years, I would be looking for a specific cause.
Can you give some more information?
Charles Nix