Certain calendar events not syncing

I'm syncing my iphone to my Google account, and for the most part it works. However, in this very specific situation, it's failing and I can't figure out why.

I've got Calendar enabled in Settings → Mail, Contacts, Calendars → Gmail, and on both a 3rd party calendar app and the native iphone Calendar, I can see all my Google calendars, create events (in the native app), and get them to sync to Google so I can see them on my desktop.

Then, I made a restaurant booking via opentable.co.uk in safari on my phone. At the end of the booking process, it offers 'Calendar' as a link, and when clicked it opens a 'new event' kind of form - presumably an add-in from the Calendar app. I can see the event has all the right details, I can choose a calendar to add it to from the list of Google calendars, and it appears in the native iphone calendar just fine. But no amount of syncing / refreshing will get this event onto my google calendar so I can see it on my desktop.

If I create a test event right next to the restaurant booking in the local Calendar app, that syncs fine. I can edit the restaurant booking in Calendar, move it to a different Google calendar, etc. But it stubbornly stays only in Calendar, not syncing to Google.