I have the same issue. I'm using it on an iPad (2nd Generation) but thing is--I've merged like 5 full audio tracks already? You know, man? It's crazy. This was 2 1/2 years ago so not sure but hoping my adding a reply will get some action on this question--I believe it's as pertinent as any other. Honestly, I will go out on a limb and say that for some reason I feel like it has something to do with whenever I add a "Sampler" track. But again, unsure. I'm going to try various ways, like, merge only 2 at a time, etc. and if I get any answers or self-feedback, I'll let you know if 2 years later you're still curious...
But I'm sure there's going to be someone else and that's why I felt I had to leave a comment.