it fixes the issue I think that with the way you have your models you would have to run a query on the Confirmation model to determine whether there exists a confirmation for a particular booking. But why have a separate confirmation model at all? Try just moving the relevant field into the booking model:
user = models.ForeignKey(CustomUser, null=True, default='', on_delete=models.CASCADE)
expert = models.ForeignKey(CustomUser, null=True, default='',on_delete=models.CASCADE, related_name='bookings')
title = models.CharField(max_length=200, default='Video call with ..', null=True)
start_time = models.DateTimeField('Start time', null=True)
end_time = models.DateTimeField('End time', null=True)
notes = models.TextField('Notes', blank=True, null=True)
is_confirmed = models.BooleanField(default=False) # just this field since you already have an expert.