Timestamps not set when loading fixtures for has_and_belongs_to_many join tables

I'm using Rails 2.3.5.

When I load fixtures, Rails does not set the created_at and updated_at timestamps for has_and_belongs_to_many (habtm) join tables (the column values are nil). The migration file includes t.timestamps and the database table contains the two timestamp columns. If I explicitly include created_at and updated_at in the fixture file, it sets those values. I don't have to do this with other fixture files; Rails automatically sets the timestamp values. I'm using rake db:fixtures:load to load the data into my development database.