Initialization issues are not all that uncommon - even in DAL A software. You can get inadvertent 'race' conditions happening during initialization where tasks don't happen in the order that was planned (sometimes inconsistently, depending on some of the other external conditions during initialization). This can get particularly tricky when some of the interfacing systems may not always be alive yet during initialization. If they are short on throughput margin it can be even worse because you can't afford the processing necessary to do multiple validity checks.
These issues can also be hard to find during development and rig testing because you often have simulations of the interfacing systems, rather than the actual systems, and the simulations may not completely mimic the initialization characteristics of the actual systems.
We ran into an initialization issue with some DAL A FADEC software after it had been flying around in service for over 20 years that prevented the normal channel alternation logic from working properly during engine start. After we figured out what was happening, it turned out the issue had been there - latent - from day one. But it took a change to an interface to bring it to the surface.