Proxy value is not empty but the apt cache. Try it in an extra task.

roles/lmn_kde/tasks/main.yml

@@ -1,16 +1,4 @@
 ---
-- name: preseed packages
-  ansible.builtin.debconf:
-    name: ttf-mscorefonts-installer
-    question: "{{ item.q }}"
-    value: "{{ item.v }}"
-    vtype: string
-  loop:
-    - q: msttcorefonts/dlurl
-      v: http://livebox.pn.steinbeis.schule/mscorefonts/
-    - q: msttcorefonts/http_proxy
-      v:
-
 - name: Install desktop EDU packages and some more
   apt:
     name:
@@ -48,7 +36,6 @@
       - dia
       - vym
       - tree
-      - ttf-mscorefonts-installer
       - sqlite3
       - sqlitebrowser
       - neovim
@@ -64,6 +51,7 @@
       - sigrok
       - sigrok-cli
       - codeblocks
+    autoremove: true
     state: latest
 
 - name: Add {{ ansible_distribution_release }}-backports
@@ -84,6 +72,23 @@
   when: ansible_distribution_release == 'bullseye'
 
 
+- name: Preseed ttf-mscorefonts-installer
+  ansible.builtin.debconf:
+    name: ttf-mscorefonts-installer
+    question: msttcorefonts/dlurl
+    value: http://livebox.pn.steinbeis.schule/mscorefonts/
+    vtype: string
+
+- name: Install fonts from local mirror
+  apt:
+    name:
+      - ttf-mscorefonts-installer
+    state: latest
+    autoremove: true
+  environment:
+    http_proxy: ''
+
+
 - name: Enable splash screen
   replace:
     dest: "/etc/default/grub"