nix-shell for `contentAddressedByDefault = true;` sets seemingly incorrect `$out` value

[trofi]

Tried to create a shell to debug package SIGSEGV and found I can't run install phase for it:

# good:
$ NIX_BUILD_SHELL=$(which bash) nix-shell -A hello
[nix-shell:~/nm]$ echo $out
/nix/store/d916b1dpw6k0207a9ll0x48bdpas0zmz-hello-2.10

# bad:
$ NIX_BUILD_SHELL=$(which bash) nix-shell --arg config '{ contentAddressedByDefault = true; }' -A hello
[nix-shell:~/nm]$ echo $out
/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9

Looks unintentional.

Cc @regnat

nix-env --version output

nix-env (Nix) 2.4pre20210922_bcd73eb

[apo-llonx]

I'm facing same symptoms while using nix (Nix) 2.5pre20211007_844dd90 and trying to build NixOS configuration flake. In case example cases could provide any helpful hints, different files that were reported while halting the builds were a custom kernel config, and one of etc files of iproute2 package. The tracebacks looked very similar and both started around the fileContents function from nixpkgs/lib/strings.nix.