It’s not unusual for there to be some slowdown after all of the tissue settles down. Sometimes it’s because of scar tissue buildup, sometimes it’s your prostate, sometimes they have no idea. It’s not a concern unless it drops below a certain point. If your last test showed 25ml/sec and it’s suddenly 15ml/sec, that’s actually not a big deal. 15 would still be considered a great output. If it’s under 5, well that might be more worrisome.
It’s probably not a problem, since nearly everyone who’s had a urethroplasty has had the same concern, but it would be a good idea to have it checked out, if nothing else than to ease your mind. Good luck, and let us know how it goes.