Been gone awhile, but back now. I too am a Federal LEO with Customs and Border Protection as a K-9 Officer. I get a gun and a pair of jaws to protect me!
However, I used to be a San Diego Sheriff’s Deputy, and I didn’t ever cite anyone specific for a seatbelt violation on that alone. I may of rolled it into the citation for other reasons on a stop, but I can’t recall ever stopping a person just for that.