Can you please check your Google Calendar settings for “Add invitations to my calendar”? If you have it set to “Only if the sender is known”, test it with the other two?
If we set our Google Calendar settings to “From Everyone” the Android to iOS invitations (do now sync ok) to Google Calendar online 
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If we set it to “When i respond to the invitation” the Android to iOS invitations (do not sync) to Google Calendar online.
If we set it to “Only if the sender is known” the Android to iOS invitations (do not sync) to Google Calendar online.