I was hit by this as well, when installing dictionaries to  the
non-default profile.

As Ludo pointed out:

--8<---------------cut here---------------start------------->8---
$ aspell config
...

# dict-dir (string)
#   location of the main word list
# default: <$ASPELL_DICT_DIR|home-dir/.guix-profile/lib/aspell> = /home/ambrevar/.guix-extra-profiles/default/default/lib/aspell
dict-dir /home/ambrevar/.guix-profile/lib/aspell
...
--8<---------------cut here---------------end--------------->8---

Something may be wrong with our patch:

--8<---------------cut here---------------start------------->8---
+#undef DICT_DIR
+#define DICT_DIR "<$ASPELL_DICT_DIR|home-dir/.guix-profile/lib/aspell>"
+
--8<---------------cut here---------------end--------------->8---

I had a quick glimpse at Aspell's source code but nothing obvious came
out :p

Maybe ask upstream?

-- 
Pierre Neidhardt
https://ambrevar.xyz/