I'd think this is the expected behavior, in case macro expansions are needed in
the comments, though it's seldom used.  Does escaping the macro name using the
form "%%{macro_name}" not give the desired text?

A .spec line like

# %{_configure} doesn't work

will expand to

# [goobledygook]
[more blah] doesn't work

and this will yield a error message from sh ("EOF found while looking for `''"),
which is at first sight entirely unrelated to the  .spec file.  

The content of comments should be ignored, and expanding in a comment should
have no effect, by definition, as far as I'm concerned. Either that or, all %{}
macros should be on a single line.

If this is a useful feature, then I'm little red ridinghood.

And I'm the Big Bad Wolf :-)

Add the extra % ...

I'm sure the workaround you've mentioned will work, but doesn't it seem odd to 
not ignore the comments?  perhaps rather than being marked resolved, this should
be changed to an enhancement req, marked low priority, and shuffled to the
bottom of the pile??