fix: write w:next at XML level so self-referential next styles persist

python-docx's next_paragraph_style setter drops self-references (WA Nrml
Pgr -> itself, WA TOC -> itself), which would make Word continue those
paragraphs as Normal; the test now asserts the w:next element.

tests/test_formatter.py

@@ -959,13 +959,22 @@ class WAStyleFamilyTests(unittest.TestCase):
 
     def test_wa_style_bases_and_next_styles(self):
         st = self._format().styles
+
+        def next_id(sid):
+            # Assert the w:next ELEMENT exists — the python-docx getter falls
+            # back to the default style when w:next is absent, which would
+            # otherwise let self-referential next styles pass vacuously.
+            el = st[sid].element.find(qn("w:next"))
+            return el.get(qn("w:val")) if el is not None else None
+
         self.assertEqual(st["WA Chapter"].base_style.name, "Heading 1")
         self.assertEqual(st["WA Section"].base_style.name, "Normal")
         self.assertEqual(st["WA Thkng"].base_style.name, "WA Nrml Pgr")
-        self.assertEqual(st["WA Chapter"].next_paragraph_style.name, "WA NChp Pgr")
-        self.assertEqual(st["WA Section"].next_paragraph_style.name, "WA NChp Pgr")
-        self.assertEqual(st["WA NChp Pgr"].next_paragraph_style.name, "WA Nrml Pgr")
-        self.assertEqual(st["WA Nrml Pgr"].next_paragraph_style.name, "WA Nrml Pgr")
+        self.assertEqual(next_id("WAChapter"), "WANChpPgr")
+        self.assertEqual(next_id("WASection"), "WANChpPgr")
+        self.assertEqual(next_id("WANChpPgr"), "WANrmlPgr")
+        self.assertEqual(next_id("WANrmlPgr"), "WANrmlPgr")
+        self.assertEqual(next_id("WATOC"), "WATOC")
 
     def test_wa_chapter_outline_level_and_automatic_color(self):
         el = self._format().styles["WA Chapter"].element

umbraformat/formatter.py

@@ -374,10 +374,16 @@ def _create_wa_styles(doc):
             color_el.set(qn('w:val'), props["color"])
 
         if "next_style" in props:
+            # Write w:next at the XML level: the python-docx property setter
+            # silently drops self-referential next styles (WA Nrml Pgr ->
+            # itself, WA TOC -> itself), which would make Word continue those
+            # paragraphs as Normal instead of the WA style.
             try:
-                style.next_paragraph_style = doc.styles[props["next_style"]]
+                target = doc.styles[props["next_style"]]
             except KeyError:
-                pass
+                target = None
+            if target is not None:
+                style.element.get_or_add_next().val = target.style_id
 
         if props["type"] == "paragraph" and style.paragraph_format:
             if "outline_level" in props: