It's hard to give an idea of how to repeat the bug because I'm not excactly sure when it happens, I just know that it happens.
So on subassemblies that were originally mirrored, after continuing to build complex builds with multiple mirrored subassemblies, other subassemblies will become unmirrored and will instead become individual parts again. I'm not sure if this happens after many saves or after exiting the game or both.
@AndrewGarrison Oh I see. thank you for clarifying. The other issue I encountered was also a mirrored group would eventually become un-mirrored and just a collection of parts, and the original mirror option would cease to exist. The game eventually un-mirrors parts and all parts attached to the root and they instead become regular un-mirrored parts, I'm not sure if this is also intentional or not. It's not a big deal except when editing an originally mirrored part or adding to the root part, the mirrors no longer take on the editing or additions to the root.