Explain the problem.
test.md:

[bracketed span]{}

command:

pandoc test.md -f markdown -t markdown

output:

bracketed span

expected output:

[bracketed span]{}

When test.md is modified as follows:

[bracketed span]{.class}

(i.e. the Attr is non-empty), then the output is as expected:

[bracketed span]{.class}

It should also be noted that in case of an empty Attr, the Markdown parsing works as expected:

$ pandoc test.md -f markdown -t native
[ Para
    [ Span
        ( "" , [] , [] ) [ Str "bracketed" , Space , Str "span" ]
    ]
]

Therefore, this is an issue with the Markdown writer.

Pandoc version?
3.6.4