Why is the first line of source code not correctly shown when importing markup Markdown?

[Robert Graf]

In Confluence, on a page, when I insert markup with Markdown (type on left window or paste from a .md file), e.g.

This is source code

    Line1
    Line2
    Line2

the preview and final result is:

This is source code

Line1
    Line2
    Line2

First line is normal paragraph and Line1, Line2, Line3 are style preformatted but for Line1 the first 4 spaces are missing/lost.

Finally I'm also not happy that it will be preformatted and not surrounded by a "code block (plain text)" or the "No format" block.

Is there a way to solve the problem and make Markdown import more consistently correct?

Used Confluence 6.15.9, Chrome 78.0.3904.97, also tried IE 11.864.17763 under Windows 10, 1809, 17763.864

Thanks!