I have had my j pouch almost 35 years and have this problem. I thought it was because I used so many muscles to control my loose stools that I'm unable to fully empty my bladder.
I had surgery for urethral diverticula about 10 years post OP and testing a few years ago that showed I was not emptying my bladder.
I just assumed it had to do with j pouch surgery just like all the other complications I've had. I've learned to live with it.🙂