We don't even know if the flaw is in the battery itself. Maybe it's in the charging circuitry, or even the software controlling the charging cycle. Personally in a situation like this I wouldn't care where the flaw was, I'd want a different replacement device anyway. Especially if even the device manufacturer apparently can't figure out how to make these things safe.

Finally, one theory is that Samsung designed the battery cells to be too thin. Since removable battery designs reduce the amount of space available for a battery, the constraints on the battery size would be even greater, conceivably making an issue like this even more likely.