Stream doesn't preserve the order after grouping

I have a List name availableSeats I am sorting and grouping by the blockIndex property like below:

availableSeats.stream()
                .sorted(Comparator.comparing(SeatedTicketAssignment::getBlockIndex))
                .collect(Collectors.groupingBy(SeatedTicketAssignment::getBlockIndex))
                .forEach((block, blockAssignments) -> {
                     //Rest of the code
                } 

The problem is that the result of grouping by is not sorted by the blockIndex.