Wrong filler rod??? That's kind of an off the cuff remark. Is there a piece of the extruded or machined part broke off in the lower tube? If not that sucks. That would make it basicaly just a butt weld and that's a very weak weld joint, especially when welding a thick part to a thin wall tube and in aluminum too boot. There is a Redemption with a simular break also on this forum and Felt stood behind it's product and made right with the owner. I'm not sure how long the waranty on the frames are or if they are to original owner only though. What year is this Compulsion? I have an 08 Compulsion 2 and it doesn't have this feature but my 08 Redemption does.
Off the cuff remark? I didn't say it to be spiteful towards Felt. Look at where it broke. Right in the middle of the welds. Usually that is a indication of using the wrong filler rod, low heat (poor penetration), too high of heat with the wrong filler rods, or bad heat treating. I can't comment on Felt aluminum bike quality since I'm riding one of their full carbon lineup.
I heard nothing but good things about Felt and I'm willing to bet they will replace that frame no question ask.
I'm curious to see if the machined part has broken off inside the seat tube. If not that sucks, it's a butt connection. As a welder fabricator myself I still say it's most likley not incorrect filler rod. In a job shop maybe, as your weld different items all the time. But this is very less likely to happen in a production manufacturing facility where the weld stations are set up to do the same type of welds on the same types of materials day in and day out.