alert time of an event that has set travel time is wrong when using iCloud sync

When I add a new event on OS X, e.g. “12pm have lunch and travel time is 1 hour, alert when I need to leave”.

On my mac, it is correct. It shows that “event time: 12pm, travel time: 1h, alert: when need to leave (that is 11am)”.

On iCloud website, it is also correct. It shows that “event time: 12pm, travel time: (not supported), alert: before the event time 1h (that is 11am)”.

But on my iPhone, it is wrong. It shows that “event time: 12pm, travel time: 1h, alert: before the time I need to leave 1h (that is 10am)”.

By the way, OS X version: 10.10.3 (14D113c) (latest version)

IOS version: 8.3 beta4 (latest version)