Well well well, doesn't this sound familur. lol.
I played this game with my Corgi for about ohhh... 7 months?
Ran every test in the book, and other than a few spores, nothing was wrong on paper.
Here's what we came up with a few thousand bucks later.
My Corgi has a problem processing carbohydrates. His body treats them like toxins, and can not absorb them, and it literally leaves "junk" rotting in his system, making for perfect breeding grounds for bacterias. Even the good bacterias in extreme excess can cause problems like diarhea and even vomiting depending on where in the system they are "rotting." Each time we changed foods, and carbohydrate sources, stools would firm up for a couple days but every time, as the new source took over his system, the diarrhea would come back just like before. There is no real test that can confirm this, it's kind of a "when-everything-else-checks-out" kind of diagnosis. I don't even know the real name of it, or if it even has one, but when my vet explained it, he paralelled it to SIBO. It is much like SIBO, but generally a dog will never grow out of it or "get better" after a round of antibiotics like many with SIBO do. It is going to flare up every time any real amount of carbohydrates is ingested, and as long as carbohydrates are a large part of the diet, you will never see improvement. It is a cronic thing that requires treatment throughout the life of the dog. Now for the even worse part. If you just shrug it off and say "oh well, diarrhea forever" then it acn get worse and worse, and eventually wear their immune system down (from constantly fighting the carbs like toxins) SO much that it will shut down, and be unable to fight the toxins, not to mentio any illness or infection.
THE GOOD NEWS: It's totally manageable. A vet will recommend: steroid shots as needed (mine was every two weeks) the highest dose of antibiotics that can be given (mine told me to watch for lameness, uncoordination, dragging of hind limbs, etc. due to the high dose) and a perscription diet. The treatments are to keep the carbs in check that are in EVERY dog food, including whatever RX diet is perscribed.
WHAT MADE SENSE TO ME: I chose to skip out on putting my one year old puppy on such extreme treatment and took matters into my own hands. I knew that dogs had no real need to carbohydrates in their diet. (they use fats like we use carbs) so it made more sense to me to simply eliminate the carbohydrates, skip the control treatments, and see how it worked. I put Grisom on a prey model raw diet after learning everything about it here, and he has now had solid poo for four weeks. I get that some people don't want to go raw, and this is the kibble section, so I won't go on about it. A home prepared diet (raw or cooked) is the only way to eliminate the carbohydrates- NO dog food caters to this issue, none.
I am not saying for sure this is what is wrong with your dog, it just sounds exactly like what I went through with my Corgi, Grissom. This is what worked for us, this is what gave us results, and I'm simply sharing that. I am not a vet, I can't diagnose, just sharing what we went through.
ETA: Through Grissom's 7 month ordeal, he always had great energy level, healthy coat, teeth, gums, everything checked out fine. He JUST had diarrhea. It was tough to keep weight on him at times, but not always and this wasn't a symptom until after about 4 months of diarrhea. He had small amounts of diarrhea many many times each day. He never really acted sick at all except for a few select days through the whole process.
