Boost 1.76 and 1.77 (beta) don't create static libraries on macOS with build-type=complete

Boost 1.76 and 1.77 Boost.Locale doesn't create the static libraries (s, sd flavors) with build-type=complete on macOS (Mojave, Bug Sur, both arm & intel). I'm using boost.locale.icu=off boost.locale.iconv=on parameters and you can see the complete b2 command below.

On Boost 1.75 it works and creates the following libraries:

libboost_locale-mt-a64.a
libboost_locale-mt-a64.dylib
libboost_locale-mt-d-a64.a
libboost_locale-mt-d-a64.dylib
libboost_locale-mt-s-a64.a
libboost_locale-mt-sd-a64.a

On Boost 1.76 and 1.77 does not create the static versions (sd & s flavors), but just the dynamic versions:

libboost_locale-mt-a64.a
libboost_locale-mt-a64.dylib
libboost_locale-mt-d-a64.a
libboost_locale-mt-d-a64.dylib

The complete b2 command used is the following:

./b2 --prefix=./bld --stagedir=./bld/stage --build-dir=./bld/int -j16 --build-type=complete --layout=tagged runtime-debugging=off debug-symbols=on address-model=64 cxxflags="-stdlib=libc++ -std=c++11 -fPIC" linkflags="-stdlib=libc++ -std=c++11 -fPIC" --without-python --without-iostreams boost.locale.icu=off boost.locale.iconv=on install > ./build.log

Start by running the provided b2 command for Boost 1.76 or 1.77 and compare its staged output with Boost 1.75 on macOS. Trace the Boost.Locale build rules involved in the s and sd variants; done means build-type=complete produces both static libraries alongside the dynamic variants.

Written by the indexing model from the issue text.