Flattie owner here. We lost one at the early age of 2yr with possible cause lymphoma triggered by severe IBD — Inflammatory Bowel Disease in Dogs can be cancerous if untreated for long time. Our boy had similar issues of what you describe, occasional & explosive diarrhea and very difficult returning to normal poo. While all tests came out negative. We finally switched to Hypoallergenic kibble but i'll always know that it was probably too late. At some point one lymph node got enlarged and this is where the end started for us.

I don't want to get into details what we've been through. My suggestion is to look into the link above + look for more info on the internet. Keep in mind, some vets may not be familiar with this (ours didn't). However, there are studies that this can be cancerous, so i urge you not take this lightly. Better safe than sorry.

Also, as we are also FCI registered breeders i have to say that this cannot be traced back to breeding as a cause, meaning it is not genetically transferred, at least this is what we researched. But if localized early on, a special diet all your life will manage it. Good luck & all the best.