The only thing I can think of with a sudden die-off is that avos are /extremely/ salt sensitive. Having it in a container without buffer from the surrounding soil could contribute (if that’s where yours is at). So if you’re fertilizing it too heavy in those conditions, you get burning in the roots, translates to systemic root failure etc. And then the very feast-famine nature of container moisture might be giving them wet feet that they don’t like. (Not a lot of varience available for the plant to root in to keep oxygen coming in, not a lot of places or depth to spread into etc).

My understanding with avos is that they do good with low irrigation, lower nutrients, and do well with like a sand-clay almost? They’re weird.

Without seeing anything else in-person I’d guess too much nutrients and also root rot. See how much you can dial in the soil type, back off on fert etc. Might grow slower because of that, but, that might just have to be what it is IG. Remember to water only when the plant shows wilting from needing it, make sure you got sharp drainage.

Not an expert but I work with plants! Hopefully any of that helps!!!