Back in the 1950's the first horse I owned had string halt. She'd always had it (she was 14 when I got her) and it never bothered her. I did all Pony Club and Horse Club activities with her, hunted her, jumped her etc., eventually retiring her and then having her put to sleep at the age of 28.
With her, and with string halt in general as I understand it, the 'movement' is always there, it doesn't come and go with relatively long 'gaps' between bouts?? I would suggest that the posters mini might have a 'catching' stifle for some reason and not actually string halt?? I agree that I would be looking for a second opinion.