Triggers lost when changing table property in SQL Server Management Studio designer
You shouldn't be doing those changes in the GUI anyway but in T-SQL it's the only way you can reliably publish changes from dev > test > prod and it's the only way you can have source control on your database structure.

That being said, this happens when you change things that require the table to be recreated and is not a bug but documented behaviour.

Please see Error message when you try to save a table in SQL Server: "Saving changes is not permitted" which clearly states (emphasis mine):


  This problem occurs when the Prevent saving changes that require the
  table re-creation option is enabled, and you make one or more of the
  following changes to the table:
  
  
  You change the Allow Nulls setting for a column.
  You reorder columns in the table.
  You change the column data type.
  You add a new column.
  
  
  When you change a table so that you alter the metadata structure of
  the table, and then you save the table, the table must be re-created
  based on these changes. This may result in the loss of metadata and in
  a direct loss of data during the re-creation of the table. If you
  enable the Prevent saving changes that require the table re-creation
  option in the Designer section of the SQL Server Management Studio
  (SSMS) Options window, you receive the error message that is mentioned
  in the "Symptoms" section.


The work-around in that article is exactly what you should be doing:


  To work around this problem, use Transact-SQL statements to make the
  changes to the metadata structure of a table.