Created attachment 154468 [details]
before the merge

Created attachment 154469 [details]
After the merge

Thank you for reporting the bug.
it seems you're using an old version of LibreOffice.
Could you please try to reproduce it with the latest version of LibreOffice
from https://www.libreoffice.org/download/libreoffice-fresh/ ?
I have set the bug's status to 'NEEDINFO'. Please change it back to
'UNCONFIRMED' if the bug is still present in the latest version.

HI,

I installed version 6.3.1.2 (last)

The issue seems the same.

With combine function I have the line attribute and the line still on my drawing
With merge function I don't have the line attribute and the line desapear.

Thank you for your help.

Please attach a sample document, as this makes it easier for us to verify the bug. 
(Please note that the attachment will be public, remove any sensitive information before attaching it. 
See https://wiki.documentfoundation.org/QA/FAQ#How_can_I_eliminate_confidential_data_from_a_sample_document.3F for help on how to do so.)

Created attachment 154534 [details]
example

Attached the example file

HI,

the line disappear because the "no line" is set for the form after the merge.
The issue is solved when I set "no line" to a type of line.
it would be nice if the line attribute was kept after the merge.

Thank you for your help.

Merging is a complicate process: the forms to be merged, which are custom shapes, are transformed into one closed Bézier shape, consisting obviously of may Bézier curves. In my test, which looked much like yours, I got, for example, 41 coordinates for the rectangle and 97 for the ellipse (if I interpret the fact correctly that there are two arrays). I think there are as much as 41 for the rectangle because if you look closely at it, you see its corners are rounded; for the ellipse agreat number of segments is no surprise. And there still is a so-called metafile having 36564 lines, I assume it contains information about the Bézier curves used, but I don't really know.
Now in your example both shapes had the same attributes, but the general case is that they may differ. The result of merging is one object, having one field for each attribute, so the solution to set the attribute to the default value seems logical.
On the other hand, area fill is handled differently: it seems that the colour of the object lying farthest back (menu item : Send to Back, Send Backward, as a default set in order of creation) is used to set the colour of the new object. So one could argue that the same procedure should apply to the line attributes, which in your example would exactly what you wish.
But then I would regard it as an enhancement request (set Importance: enhancement). If you do so, I will confirm the bug.

(In reply to Gerhard Weydt from comment #9)
> Now in your example both shapes had the same attributes, but the general
> case is that they may differ. The result of merging is one object, having
> one field for each attribute, so the solution to set the attribute to the
> default value seems logical.
>
> On the other hand, area fill is handled differently: it seems that the
> colour of the object lying farthest back (menu item : Send to Back, Send
> Backward, as a default set in order of creation) is used to set the colour
> of the new object. So one could argue that the same procedure should apply
> to the line attributes, which in your example would exactly what you wish.
> But then I would regard it as an enhancement request (set Importance:
> enhancement). If you do so, I will confirm the bug.

Let's ping UX

Merging takes the color of the first object, no line color/size but shape (try with a large dotted line). Drawing style preserves area fill settings but not what's defined for lines, the style has to be applied again.

As Gerhard pointed out, the merge process transforms the shapes into a bezier curve. And this object can have lines, makes sense to take the property from the first object.

Hello,

same on version 7.3.2.2.

When I merge two objects with a line the new object have no line.

Thank you for your help.