When joining several combined lines they change shape. How do I stop it

I am only a very occasional user of CorelDraw and I get stuck easily. I have now hit on a snag I can't find my way out of so am asking for help.

What I am trying to design is a decal for a model locomotive that includes its main body colour, red, decorative lining, in yellow, and a black border. The original source of the drawings is a CAD program (that has the full design of the model in it) exported as a DXF or DWG file (i've tried both) and then imported into CorelDraw X8

What I have done is

• select various lines from the imported layer and moved them to named layers, inner, outer and border.

I now want to join all the inner lines so the shape will fill

• Selected all the lines, right clicked and combined them
• Used the shape too to move the ends onto each other so they join
• When the last one is joined the shape fills with colour

As you can see from the examples below....

BEFORE
At this stage the image is 8 separate lines imported as a DXF file. They are then all selected and the combine command used

AFTER
With the Shape tool selected I start joining the lines so they are one continuous line. As I join the right hand vertical to the top right curve the bottom left curve changes to an S

EXAMPLE
Although this is a different part it does show what I want to achieve and why the S is wrong

This may be a red herring but it seems to be the panels that have all four corners scalloped that has the issue. I'd be grateful for an understanding of why its doing this, how i stop it or how i cheat if its always going to do this.

Finally, my own experience of being asked a question about something is i am inquisitive about the wider scope so for people like me I enclose this picture of the real locomotive. The model is currently painted black but a friend is able to produce waterslide decals to the aim is to produce decal panels that have the red body colour, the yellow lining and a black border and to slide them onto the black loco.  Its a model version of the vinyl wrap that they use to change the livery of modern trains each time the owner changes.

Thanks in anticipation

Paul