I've done some for fuel estimation of freighter jets (not quite avionics but close enough to get a sense for the development processes) and the amount of rigor involved in that one project made me a better developer for the rest of my career. Was it slow? Yes, it was very slow. A couple of thousand lines of code, a multiple of that in tests for a very long time compare to what it would normally take me.
But within the full envelope of possible inputs it performed exactly as advertised. The funny thing is that I'm not particularly proud of it, it was the process that kept things running even when my former games programmer mentality would have long ago said 'ship it'.
Some things you just need to do properly, or not at all.
It wouldn't surprise me to find doing the same kind of certifications for complex avionics software to be the same.
I would presume there's the same issue as parent said: