ExternalProject logging mystery

Code
1

Look at this:

ExternalProject_Add(boost
	PATCH_COMMAND echo "using gcc : ${CMAKE_SYSTEM_PROCESSOR} : ${CMAKE_CXX_COMPILER_CMD} $<SEMICOLON>" > <SOURCE_DIR>/user-config.jam
	LOG_PATCH OFF
	...
)

This works well.

Now look at that (LOG_PATCH ON):

ExternalProject_Add(boost
	PATCH_COMMAND echo "using gcc : ${CMAKE_SYSTEM_PROCESSOR} : ${CMAKE_CXX_COMPILER_CMD} $<SEMICOLON>" > <SOURCE_DIR>/user-config.jam
	LOG_PATCH ON
	...
)

That goes wrong and does not produce user-config.jam (without any error).

Any ideas what’s the problem?