lib/libc.so.7: chflags: Operation not permitted

I built world && kernel from within a jail. When I attempt to
perform an install to DESTDIR, I receive the following:

releng12:/usr/src # mkdir -p /REL
releng12:/usr/src # make installworld DESTDIR=/REL
...
--------------------------------------------------------------
>>> Installing everything
--------------------------------------------------------------
cd /usr/src; make -f Makefile.inc1 install
make[3]: "/usr/obj/usr/src/compiler-metadata.mk" line 1: Using cached compiler metadata from build at 12current.localhost on Fri Feb 14 20:57:38 PST 2020
===> lib (install)
===> lib/csu (install)
===> lib/csu/amd64 (install)
install  -o root -g wheel  -m 444 crt1.o crti.o crtn.o Scrt1.o gcrt1.o /REL/usr/lib/
===> lib/libc (install)
install  -C -o root -g wheel -m 444   libc.a /REL/usr/lib/
install  -C -o root -g wheel -m 444   libc_p.a /REL/usr/lib/
install  -s -o root -g wheel -m 444   -fschg -S  libc.so.7 /REL/lib/
install: /REL/lib/libc.so.7: chflags: Operation not permitted
*** Error code 71

Why isn't it permitted. I've not imposed any special permissions on, or
within the jail' folder. Nor within the jail itself, and I've been doing this
sort of thing within jails for years, and have never had this problem.

Thanks!

--Chris