That style of heading is created by the \synopsis command and I think the lack of tag is intentional (it doesn't start a new section). See also the <functional> synopsis in [function.objects], the <tuple> synopsis in [tuple.general], the <variant> synopsis in [variant.general] etc. etc.

[optional.syn] is inconsistent and doesn't use the \synopsis command:

\rSec2[optional.syn]{Header \tcode{<optional>} synopsis}

Also [memory.syn]

\rSec2[memory.syn]{Header \tcode{<memory>} synopsis}

And [meta.type.synop]:

\rSec2[meta.type.synop]{Header \tcode{<type_traits>} synopsis}

Use of \synopsis is optional. Lots of times we use real, numbered headings, exactly because getting a referenceable section name is useful. For the new C library synopses we tried sticking to a pattern by which the stable name for the synopsis of <cfoo> is [cfoo.syn].

See also Issue #566.

The original issue was fixed with #1213.