Surprising expansion of variadic GNU C preprocessor macros in the presence of the ## operator [duplicate]

If we define a macro

#define M(x, ...) { x, __VA_ARGS__ }

and then use it passing itself as an argument

M(M(1, 2), M(3, 4), M(5, 6))

then it expands to the expected form:

{ { 1, 2 }, { 3, 4 }, { 5, 6 } }

However, when we use the ## operator (to prevent dangling comma from appearing in the output in the case of the single argument invocations, as documented in the GCC manual), i.e.

#define M0(x, ...) { x, ## __VA_ARGS__ }

then the expansion of arguments in

M0(M0(1,2), M0(3,4), M0(5,6))

seems to stop after the first argument, i.e. we get:

{ { 1,2 }, M0(3,4), M0(5,6) }

Is this behavior a bug, or does it stem from some principle?

(I have also checked it with clang, and it behaves in the same way as GCC)