Well, with a fairly recent guix:

Generation 78   Mar 07 2024 12:51:33    (current)
  guix d29e5a8
    repository URL: /home/vagrant/src/guix
    branch: master
    commit: d29e5a83e887cd2f4f459a12cbbfc40c77e55ce2

gnucash 5.5 is now still not reproducible, but apparently for a possibly
different reason, the share/doc directory seems to include a versioned
and/or unversioned directory:

guix challenge --verbose --diff=diffoscope gnucash
guix challenge: warning: could not determine current substitute URLs; using defaults
/gnu/store/jbqhcipnb5fi52q10iaw387jhcq64dxs-gnucash-5.5 contents differ:
  no local build for '/gnu/store/jbqhcipnb5fi52q10iaw387jhcq64dxs-gnucash-5.5'
  https://ci.guix.gnu.org/nar/lzip/jbqhcipnb5fi52q10iaw387jhcq64dxs-gnucash-5.5: 01fx1b44nmfxn0rclxwcmz43nxz87gqn2lhk1918czr7na3hfzvg
  https://bordeaux.guix.gnu.org/nar/lzip/jbqhcipnb5fi52q10iaw387jhcq64dxs-gnucash-5.5: 1wxbcwqhj5fj7v38x7m6bdgays735pbghzkgsymggcqr9awvqxhx
 bordeaux.guix.gnu.org  10.0MiB                                                           2.6MiB/s 00:04 ▕██████████████████▏ 100.0%
--- /tmp/guix-directory.jW05dk
+++ /tmp/guix-directory.vaUmoD
│   --- /tmp/guix-directory.jW05dk/share
├── +++ /tmp/guix-directory.vaUmoD/share
│ │   --- /tmp/guix-directory.jW05dk/share/doc
│ ├── +++ /tmp/guix-directory.vaUmoD/share/doc
│ │ ├── file list
│ │ │ @@ -1 +1,2 @@
│ │ │ -gnucash
│ │ │ +gnucash
│ │ │ +gnucash-5.5

1 store items were analyzed:
  - 0 (0.0%) were identical
  - 1 (100.0%) differed
  - 0 (0.0%) were inconclusive