Battery Share turns off when your phone:
Starts wireless charging, like when on a Pixel Stand.
Doesn’t notice that it’s charging another device after 30 seconds.
Knows the receiving device is fully charged.
Reaches a high temperature.
Senses power transmission isn’t going well due to poor placement, a thick case, or other issues.
Turns on Battery Saver.
Edit: I thought I was going crazy because I couldn't find that source. Not sure if this implementation of the Battery Share is significantly different from stock for this info to be included on the feature page.