Race condition while creating a new partition

We use this code in order to automatically create new partitions for a partitioned table (Postgres 10.6):

begin
  insert into ;
exception when undefined_table then
  begin
    
    -- A concurrent txn has created the new partition
    exception when others then end;
    -- Insert data into the new partition
    insert into ;
  end;
end;

As far as I understand we should always have a new partition created either in current or in concurrent transaction but today we faced the problem when we failed to insert data due to a partition nonexistence for a small period of time. Have I missed something?

Thank you.

—

With best regards, Andrei Zhidenkov.