Just wanted to dip my toe in to suggest not using flashbacks to achieve what a small moment between two characters reminiscing can do. It can be a better experience but it can also be not as effective as some subtle dialogue, depends on the circumstance, and also it takes time away from developing the current story which should probably be the main focus.
The only issue is, if the purpose of bringing up the past is for characterization, without any specific 'wow' moments that recontextualize current story stuff, then ideally we would have gotten them in the first couple of chapters.
Personally I'd be happy with inferring a zombie apocalypse can make people do weird stuff, but if you want to be explicit about it i just hope you don't bend over backwards for it.
Maybe you already agree with me and aren't planning to dig any deeper into the context of the story anyway.