It's not really a new behaviour though, it's the one that you should have done all along but could put up your own work boundaries or learn to say no. The question is why the employee felt the need to do that in the first place. Then once the perceived expectation wasn't met (promotion, pay rise, that hot guy/girl in accounts) it becomes "quiet quitting".

You have to be accountable for your own work boundaries, it's not the employers problem that an employee's self control isn't functioning how it should.

Just my thoughts.