Why it happens
Microsoft Bookings checks free/busy on your default Outlook calendar only. Events in Google Calendar, in another Microsoft account, or in a secondary calendar in the same mailbox don’t count, even if Outlook shows them side by side. Microsoft’s own community moderators describe this as a known limitation, and the usual advice is to copy each commitment onto the default calendar by hand or build a Power Automate flow to do it.
How Kelbra handles it
Connect your Microsoft account in Kelbra and choose its default calendar (the one simply called "Calendar"), then connect your other calendars. Kelbra copies their events onto that default calendar, so Bookings sees them and stops offering those times. With Busy only turned on, the copies show as "Busy" with no title or details. We tested this with Bookings with me: an event added in Google disappeared from the booking page’s available times within seconds. Automatic sync is part of the paid plan; during the free trial, press "Sync now" after a change.
Fixing it
Check which calendar Bookings reads: it’s the default calendar of the Microsoft account your booking page belongs to, usually named "Calendar".
The manual fix: whenever you add something to another calendar, add a matching "Busy" event to that default calendar too. It works, but it’s easy to forget.
The automated fix with Kelbra: connect that Microsoft account and pick its default calendar when asked which calendar to sync.
Connect the calendars Bookings can’t see, such as your Google Calendar or a second Microsoft account. Turn on Busy only for any whose details should stay private.
Test it: add an event in the other calendar at a time your booking page offers, wait a few seconds (or press "Sync now" during the trial), and refresh the booking page. That time should be gone.
Questions
Why does Microsoft Bookings ignore my Google Calendar?
Bookings only checks the default calendar of your Microsoft account. Google calendars, other accounts and secondary calendars aren’t part of its availability check, even when Outlook displays them.
Can Bookings check more than one calendar?
Not on its own. Bookings with me checks your default Outlook calendar. To make other commitments count, they have to be on that calendar, either added by hand or copied there by a sync tool.
Will the people booking me see my private event details?
No. Bookings only uses free/busy to decide which times to offer. With Kelbra’s Busy only setting, the copies on your Outlook calendar don’t carry titles or details either.
How quickly does a new Google event block the slot?
In our test it took a few seconds on a paid plan. During the free trial, sync runs when you press "Sync now".