My friend has had the same thing happen to him with his laser/light combo. We're thinking it has something to do with the shorter barrel not giving enough of a foothold for the light. We've also noticed that the side grooves on the rail aren't as deep as the grooves on the laser/light. That could have something to do with it as the laser/light just may not have enough to "hold" onto when the barrel recoils. It's pretty disappointing, as you said. There's so much money to be made by any company that chooses to come up with good-quality, economical products that many of us gun enthusiasts are looking for. A good laser/light combo that actually fits snug onto an XD/XDm and a good holster that will accommodate them would be at the top of my list.
Texas CHL Carrier
NRA - Life Member
"There are millions of gun owners in this country, and not one of them will have an accident today. The only misuse of guns comes in environments where there are drugs, alcohol, bad parents, and undisciplined children. Period." - Ted Nugent
"And now," Jesus said, "take your money and a traveler's bag. And if you don't have a sword, sell your cloak and buy one." - Luke 22:36