SA's leaving without notice is nothing new. From any luxury house. Sometimes things just happen and they don't even expect it. Some SA's may tell you before they leave, some won't. It's just a part of life. Retail is a stressful business and I don't blame them if they want to move up or try something new/different.
I think it really depends on
what kind of SA you have, how long you have known the SA, and the situation he or she is in.
Years ago, one of my Dior SA's left. When I asked where he went, I was actually told that they could not tell me lol. I later figured out where he went and I think he realized how much I valued him as my SA, so when he decided to move on to his next chapter in life, he was kind enough to tell me.

I wished him the best and I hope he is doing well!
I've also had an SA who've left and told me, and was very kind and generous to give me a parting gift.

I've had SA's run into personal issues, and while they have been kind enough to talk to me about them, I do kind of see it as a sign that they may move on soon. It has happened with 2 of SAs. Nothing against them of course, I am glad that they were able to prioritize their own lives

All of these SA I have know for years, maybe on average 5-6. And they have been wonderful people to work with!
Generally though, when an SA leaves for me (knowingly or not), I usually very quickly get assigned a new SA. Doesn't necessarily mean I have to stick with them, despite getting "assigned" to a new one. Management usually figures out how responsive I am to an SA that I like vs one I'm not particularly compatible with, lol

. But I have rarely not worked out with a new SA I was assigned to, I think that only happened once.