Why are my app reviews deleted so often and quickly since iOS 10.3?
Developers can not directly remove reviews but Apple's App Store staff can.

Developers can ask for inflammatory, misleading, or concerning reviews to be removed from the App Store and Mac App Store.

When a new version of an application is released on an Apple App Store, existing reviews can be archived and removed from immediate sight. This may be the behaviour you are seeing.

Regarding time zones, Apple appear to have review teams covering all time zones. Reviews of newly submitted applications occur on weekends and most holidays. Thus it is reasonable to assume other app store services are available at all times.