When a participant declines the meeting it's still visible in the calendar and the timeslot is not made available in scheduler. We fetch the availability from the provider, not all providers treat declined events equally.
Microsoft will delete the event thus making the timeslot available.
Google will leave it in your calendar without modifying it i.e.
- busy = true
- event status = confirmed
We are working on a product improvement to check for this so that the behaviour is consistent across providers. If you have this problem please submit a support ticket and we will flag it to the feature request (TC-477).
We have no workaround currently, this is a provider limitation we are trying to improve.