Laminitis can have many causes, not just too much sugar. Friends Highland (fit, not fat) got concussion laminitis. My Sec A developed it as a side effect of Equine Metabolic Syndrome, which we only discovered she had when we blood tested her when the lami symptoms appeared. Dealt with the EMS, no lami any more. She has to be managed, but 2 years on and no recurrence.
Maybe do a faecal worm egg count before worming to see if wormer is required.
Not heard of a wormer - laminitis link, but if pony gets an upset system with the wormer, stress can bring on lami too.
Could it be you are worming at the same time the spring grass is really coming through (full of sugar)?
Just some ideas for you, sorry if you know all this already.