It's not a bug.
How threads are viewed is something each member controls themselves.
Click on "My Settings" at the top of the page.
Then click on "Settings and Options".
Then "Edit Options"
Scroll down the page until you see "Thread Display Option"
Scroll down until you see "Thread Display Mode"
Click the drop down box and use "Linear-Oldest First"
Scroll to the bottom of the page and click "Save Changes".