Meaning they said "i'll work another 3 months, but I found a new job" and their current employment said "nah, today's your last day". I've seen it happen first hand.
Of course, power dynamics being what they are, it's the worker who pays the price.
Completely OT: I absolutely know what you mean, but ugh, corporate jargon.
I do feel a bit ashamed of unironically using "sunset", although in my case it was more in the sense of "ride off into the ~". It was my last corp job (hopefully) ever.