I think the issue comes from grafts.

Without grafts, the ASDF configuration is coherent; the path for
stumpwm's sources and libraries is the same everywhere (here in
stumpwm:lib and sbcl-stumpwm-cpu):

--8<---------------cut here---------------start------------->8---
$ cat $(guix build --no-grafts stumpwm | grep 'lib$')/etc/common-lisp/*/50-stumpwm.conf
(("/gnu/store/v18hzbid3rjblz35s69w4c0gcsx9f9w9-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*)
 ("/gnu/store/v18hzbid3rjblz35s69w4c0gcsx9f9w9-stumpwm-22.11-lib/lib/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*))

(:tree "/gnu/store/v18hzbid3rjblz35s69w4c0gcsx9f9w9-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm")

$ cat $(guix build --no-grafts sbcl-stumpwm-cpu)/etc/common-lisp/*/50-stumpwm.conf
(("/gnu/store/v18hzbid3rjblz35s69w4c0gcsx9f9w9-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*)
 ("/gnu/store/v18hzbid3rjblz35s69w4c0gcsx9f9w9-stumpwm-22.11-lib/lib/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*))

(:tree "/gnu/store/v18hzbid3rjblz35s69w4c0gcsx9f9w9-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm")
--8<---------------cut here---------------end--------------->8---


With grafts, we get different paths, which probably explains why ASDF
fails to find the compiled Lisp files it is looking for:

--8<---------------cut here---------------start------------->8---
$ cat $(guix build stumpwm | grep 'lib$')/etc/common-lisp/*/50-stumpwm.conf
(("/gnu/store/wsjyjqf20ldbmgdq96h73qikmwhxv36c-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*)
 ("/gnu/store/wsjyjqf20ldbmgdq96h73qikmwhxv36c-stumpwm-22.11-lib/lib/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*))

(:tree "/gnu/store/wsjyjqf20ldbmgdq96h73qikmwhxv36c-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm")

$ cat $(guix build sbcl-stumpwm-cpu)/etc/common-lisp/*/50-stumpwm.conf
(("/gnu/store/h8i8mwsgjb96r407xqa2sf56clgy7r7c-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*)
 ("/gnu/store/h8i8mwsgjb96r407xqa2sf56clgy7r7c-stumpwm-22.11-lib/lib/common-lisp/sbcl/stumpwm"
  :**/
  :*.*.*))

(:tree "/gnu/store/h8i8mwsgjb96r407xqa2sf56clgy7r7c-stumpwm-22.11-lib/share/common-lisp/sbcl/stumpwm")
--8<---------------cut here---------------end--------------->8---