There are bugs that can be triggered only if a complex series of conditions are valid and happening at the same time, or in a specific sequence. They can be very tricky to track down (debug) and being able to replicate a bug, doesnt even mean they know the cause.
It's up to L6, once reproed a bug, to start a branch to investigate, track down, debug, solve, test and release a fix. Problem is, sometimes these bugs are affecting a very small percentage of userbase, so they probably think It is not worth it.