admin user (and ditto group member) no longer has the corresponding permissions?!

[RJVB (René Bertin)]

<preamble> >>>>
I have long used Unix permissions principles to ensure direct access to certain non-crucial "system" locations from my admin account but not from standard user accounts, simply by setting group ownership and rwx access for directories to the admin group.

I have 2 admin accounts, my regular account plus another one that I keep as close to "stock" as possible and that we shall call adplus here. NB: my "regular account" is indeed an admin-level account.

I am still running OS X (sic) 10.9.5 so anything that follows cannot be due to SIP and related "niceties". Evidently, this system hasn't seen any update love from Apple for quite some time now, and I haven't been doing any tweaking myself that I can think of (aka remember; uptime was 19d when I noticed my issue and nowadays that's long enough to forget details... esp. since whatever changed happened during the much longer uptime before that). I had been installing the dependencies of port:e17 (with sudo port -ns install e17) just before I noticed my issue, but I am not certain when I had exploited my "admin" trick for the last time before that.
<<<< </preamble>

The issue: when running a shell in the xterm emulator (or mrxvt) I can no longer access directories that I should be able to access because of their group ownership & permissions. Other X11-based terminal emulators (like xfce4-terminal) do not show this problem, and the adplus account is not affected either. No issues either when I connect over ssh from a remote linux host. However, any process launched from that xterm (or mrxvt) will inherit the permissions restrictions. Group membership of the affected account hasn't changed, and it can still use sudo for instance.

I am not seeing any log entries that are related to rejected access to these privileged directories, and AFAICT Apple's xattr-based ACL permissions are not involved either.

Fortunately I am no longer using xterm as my daily console emulator (I do most of my terminal/shell work under X11) but my X11 work environment was in fact set up through a single xterm, which is why I discovered the issue. (I would probably have discovered it a lot sooner if my sudo ability had been affected.)

I have no idea at how this issue can even be possible on a standard Unix, nor what the terminal emulator could even have to say over whether a child process can enjoy the full range of its intended permissions or not. But by now it's pretty clear that among the terminal emulators I tested the issue only occurs with these two. A defect ticket thus seems justified, if not only with the hope that someone more knowledgeable about OS X specifics can help understand what goes wrong and where (is Brandon Allbery still around?)