Fails to find a static library with runtime-link=shared

[hamstergene]

B2 fails to find static library (zlib) when I change runtime-link=static to runtime-link=shared:

$ ls /path/to/include/x64-debug
zconf.h  zlib.h

$ ls /path/to/lib/x64-debug
libz.a

$ ./b2 -n --reconfigure --debug-configure --with-iostreams -s ZLIB_INCLUDE=/path/to/include/x64-debug -s ZLIB_LIBPATH=/path/to/lib/x64-debug -s ZLIB_NAME=libz link=static runtime-link=static

notice: [zlib] Using pre-installed library
notice: [zlib] Condition

Building the Boost C++ Libraries.
    - zlib                     : yes [3]

$ ./b2 -n --reconfigure --debug-configure --with-iostreams -s ZLIB_INCLUDE=/path/to/include/x64-debug -s ZLIB_LIBPATH=/path/to/lib/x64-debug -s ZLIB_NAME=libz link=static runtime-link=shared

notice: [zlib] Using pre-installed library
notice: [zlib] Condition

Building the Boost C++ Libraries.
    - zlib                     : no [3]

[3] link=static

No other difference in command line nor in environment, other than runtime-link=static vs. runtime-link=shared.

Tried current tip of the master branch (2b06d9f20560a4f7292833fcb71d8c6a3652ca76) as well as an old version 1.71.0.

I expect C++ runtime type selection to not affect library search.

[hamstergene]

Linux, GCC v7

When zlib is not found (runtime-link=shared), bin.v2/config.log contains

set "ac-library-zlib-<address-model>64-<architecture>x86-<link>static-<target-os>linux-<toolset-gcc:version>7-<toolset>gcc" : "missing" ;

If found (runtime-link=static), then bin.v2/config.log says

set "ac-library-zlib-<address-model>64-<architecture>x86-<link>static-<target-os>linux-<toolset-gcc:version>7-<toolset>gcc" : "/path/to/lib/x64-debug" "z" "<link>shared" ;

Tried --debug-generator but there are no other mentions of zlib in the log.

[hamstergene]

Looks like this is caused by (incorrectly) passing ZLIB_NAME=libz instead of ZLIB_NAME=z, but I don't any reason why one would falsely succeed and another not.