Bran mash is a double edge sword, it helps but fed too often it will strip the natural flora of the gut, hence the reasoning for feeding probiotics. Probiotics are a good thing to give regardless. Another vet may be in order, they are not all equal in regards to their knowledge of minis (or equines for that matter). Now, this next thing is going to sound mean, and for that I appologize in advance, we do appreciate that you are seeking out input, however.......facing a problem head on prior to it rearing it's head in full force, is always a better option than being forced into a corner when your equine is in a sink or swim situation. Vet bills, yes, they suck, but every responsible owner has them at one time or another, many of us have paid thousands in bills, in payments if need be, and many have faced the decision of euthanizing if there is no end in sight, or giving a horse away if we know they have a fighting chance but can not keep paying for medical issues that may or may not arise. As already said, leaving food behind is a serious thing, if one of my horses walks away or only eats a portion of its feed, there is a serious issue! I can not emphasize this enough. Alfalfa is a natural soother, as is grass. I would cut out all feed at this point and feed soaked alfalfa or soaked alfalfa mix or soaked cubes as well as allowing the horse access to grass. There are a few things that come to mind with these symptoms.......possible sand colic, possible ulcers or ...since it is ongoing , regardless of the worming schedule, I would have to say a possible combination of parasites and/or one of the other issues. Large infestations can not be cured with one or two doses of meds, certain worms do not show up on fecals depending what stage they are in (like roundworms) and while roundworms are typically found in young or very old horses, they are also found in horses that have been compromised by illness or malnourishment. I would get a knowledgable vet involved, go to a local feed store, preferable one with a longstanding business, ask who is a good horse vet, ask if there is one at would be chosen over another, if they do not know, ask if there are any reputable stables that you can call to see whom they reccomended. There is no way around it, a good vet needs to be involved, they horse's eating behavior is not normal. I wish you the best of luck, vet bills just come with the territory, no easy way around that.