"su -m" (on Mac) preserves $HOME and $USER but not ~

[dabe]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: darwin13.4.0
Compiler: clang
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='darwin13.4.0' -DCONF_MACHTYPE='x86_64-apple-darwin13.4.0' 
-DCONF_VENDOR='apple' -DLOCALEDIR='/usr/local/Cellar/bash/4.3.42/share/locale' 
-DPACKAGE='bash' -DSHELL -DHAVE_CONFIG_H -DMACOSX   -I.  -I. -I./include 
-I./lib -I./lib/intl -I/private/tmp/bash20150827-50056-g82kmq/bash-4.3/lib/intl 
 -DSSH_SOURCE_BASHRC
uname output: Darwin triton.parabon.com 13.4.0 Darwin Kernel Version 13.4.0: 
Wed Mar 18 16:20:14 PDT 2015; root:xnu-2422.115.14~1/RELEASE_X86_64 x86_64
Machine Type: x86_64-apple-darwin13.4.0

Bash Version: 4.3
Patch Level: 42
Release Status: release

Description:

        I'm hoping this issue has been solved before, but I couldn't
        find any answers on the 'Net...

        When I "su -m", my $HOME environment variable is preserved okay
        (i.e., it remains my original home directory) but often[*] '~'
        is set to the target (root) user's home directory.

        * - It's infuriating that /occasionally/ it DOESN'T switch to
        /var/root!

Repeat-By:

        Check out this copy-paste:

                Thu Oct 29 12:32:22 dabe@triton:~> su -m root -c "bash"
                Password:
                Thu Oct 29 12:32:26 root@triton:~# echo $HOME ... ~
                /Users/dabe ... /Users/dabe
                Thu Oct 29 12:32:27 root@triton:~# echo $HOME ... ~
                /Users/dabe ... /Users/dabe
                Thu Oct 29 12:32:28 root@triton:~# echo $HOME ... ~
                /Users/dabe ... /Users/dabe
                Thu Oct 29 12:32:28 root@triton:~# echo $HOME ... ~
                /Users/dabe ... /var/root
                Thu Oct 29 12:32:28 root@triton:~# echo $HOME ... ~
                /Users/dabe ... /var/root

        Notice how my $HOME stays the same (good) but my ~ changes
        to /var/root -- and it changes AFTER I've been logged in for
        a little while!  <scratching head>

        Except for my $TTY and $PPID (and the date in $PS1, as well as
        the $ITERM_SESSION_ID), 'set' shows all my environment / shell
        variables to be identical, before and after.

        It's worth pointing out that 'su -m root -c "bash -l"' works
        fine, near as I can tell...