env -u unexpected behaviour

[Keith Maxwell]

Hello all

When I run `busybox env -u MANPAGER pwd` I get `env: invalid number
'MANPAGER'`. I'm using busybox 1.27 provided by Alpine Linux.

An easy way to reproduce this is:

    $ mirror=https://uk.alpinelinux.org/alpine
    $ curl "${mirror}/v3.7/main/x86_64/busybox-static-1.27.2-r6.apk" | tar xz
    $ ./bin/busybox.static env -u MANPAGER pwd
    env: invalid number 'MANPAGER'

For comparison I was expecting the same result as from the coreutils
implementation.

    $ env --version | head -n 1
    env (GNU coreutils) 8.25
    $ env -u MANPAGER pwd
    /

I know Alpine Linux is based on  musl-libc and looking at the source
code[1] I can see a reference to using an undocumented feature of
putenv, but that is as far as I have got.

Are you able to help please? Is this a bug?

Kind Regards

Keith

[1] https://git.busybox.net/busybox/tree/coreutils/env.c#n75