On iOS, the value of focalY is correct when State.BEGAN. However on Android, this value is always 0. Only in State.ACTIVE the focalY value is correct. Please let me know if I can help with this issue.
Interestingly enough this doesn't happen for the scale parameter.

For people having similar issues, here is a potential workaround (to adap to your needs because scale is not affected by this issue):

const pinchBegan = Platform.OS === "ios"
      ? eq(state, State.BEGAN)
      : eq(diff(state), State.ACTIVE - State.BEGAN);

Possible duplicate: #546