Most of the research around the topic quotes a study on negotiated prices at car dealerships, where women ended up with higher prices offered at the start of the negotiation and ended up with higher prices by the time the process ended.

Which is a very specific use case of a specific environment with mainly unknown opponents (as opposed to work relationships, where you typically know the boss), tight deadlines (need a car immediately) and little wiggle room (shopping around car dealerships is time-consuming, sending resumes to competitors to test the waters is less so).