I have never had PWD from sub when on kratom either. I created a poll thread a while ago asking people if they had, and of all respondents, only one said they had experienced PWD, and that person said they felt sweaty and anxious for 20-30 minutes and then fine, which is not PWD. It makes sense though, kratom is a partial agonist and so is buprenorphine. Bupe fully substitutes for kratom, in fact it is a stronger opioid. So when the mitragynine/etc gets knocked out of the receptor, it doesn't cause any issues, since bupe is stronger. The reason PWD happens is because if a full agonist gets knocked out, bupe will not substitute for it.
Also, kratom has various effects aside from the mu receptor agonism, so it stands to reason it would do a little something on bupe. I also find it does a tiny bit, but it's definitely way reduced and I don't think it's really the opiate action you're feeling. In fact, when I am coming off of suboxone, I start to withdraw some, and I try to use kratom to help and it doesn't. At least not until it's been about 2 full days, then the bupe is out of my system enough to really feel kratom again.