Download Qt5.15.1 source package (not tested from git), run configure, options used in test:

./configure -confirm-license -nomake tests -nomake examples -qt-libjpeg -qt-libpng -xcb -qt-pcre -openssl-runtime -I /home/qt/openssl-1.1.1g/include -L /home/qt/openssl-1.1.1g/lib -verbose -release -no-use-gold-linker -commercial -static -prefix /home/qt/RTA/src-5.15.1/qtbase -no-compile-examples -skip qtwebengine -skip qtwebview -skip qt3d

Build with make (-j6 used in test). Build finishes, build log does not print any errors, even if just make is run after make -j6 (which usually prints errors if multithread build 'hides' them in he log).

No assistant found in src-5.15.1/qtbase/bin, but e.g. designer is compiled and can be launched.

If same steps are done for Qt5.15.0 source package on the same machine, assistant is found and can be launched from qtbase/bin.

Edit:

5.15.1 configure.summary has:

Qt Tools:
Qt Assistant ........................... yes
Qt Designer ............................ yes