Setting a style on a selected region can set the style for the line under the selection

If you set a style on a selected region AND you selection begins or ends on the first column of the line under the selection, then the style of that line will change even though no characters on that line are selected. In V1.9.16, only the lines with visibly selected text change in style (called preset in V1). If you select to the end of the current line using End and then change style, the next line is unaffected, but including the end-of-line character will cause the following line’s style to change.

Are you talking about character styles or paragraph styles?

I’m talking about paragraph styles only. I didn’t know there were character styles, but I see them now and I tested them and they are fine. However, on another post I reported a problem with undo on styles that is a problem for both paragraph and character styles (namely that it moves the cursor to the top of the file).