Go to: Document > C/C++ Options... > Comments
Then turn off ( ) Split line comments
Also, just to note for our viewers, the appearance of incorrect indentation for the new comment is because of the indent with tabs in the browser. SlickEdit actually indents the comment correctly when you see it in your editor.
I am going to file a feature request for a future release to make "Split line comments" and "Extend line comments" work more strictly, because I can see that some people would not want the feature for comments that are not at the beginning of the line.
I will add a hot fix for 27.0.1 to add an undo step so that when you split or extend a comment, if it isn't what you want, you can undo and just get the new line.