If I understand the problem correctly, it is because FormatTail.
This method works correctly only if, since the last call of Format/FormatTail, new content was added from new line.
Use Format instead of FormatTail.
I didnt have this problem in another project about 3 weeks ago and was using formattail. I looked over and over the old code and couldnt find what I had done. So it must have been a property that allowed me to set an option to maintain current layout or something. Wish I knew what i did.
Ok, sorry Sergey but I finally worked out what it was that I had been doing...
I would start the line with AddNL() first and then append using Add() and called FormatTail when I was all done. This prevents the issue that I outlined in the original post.