Only proposals whose status is in Event Status to Include (Catering > Settings > Proposals tab > Reports) block a venue. The options are All Events, Signed and Approved Or Fully Paid Or Partially Paid, Fully Paid Or Partially Paid (the default) and Only Fully Paid. So with the default setting a proposal that is merely Sent does not reserve the venue; it starts blocking once it is partially or fully paid. The proposal you are currently editing is always excluded from the check, and so are checkouts already linked to it. Online venue orders block as soon as they are placed and keep blocking unless the order is refunded in full or canceled — the check covers checkouts in status Unpaid, Paid or Partial Refund.