[outtmpl] Pad `playlist_index` etc even when with internal formatting

Closes #7501

test/test_YoutubeDL.py

@@ -684,7 +684,7 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(id)s.%(ext)s', '1234.mp4')
         test('%(duration_string)s', ('27:46:40', '27-46-40'))
         test('%(resolution)s', '1080p')
-        test('%(playlist_index)s', '001')
+        test('%(playlist_index|)s', '001')
         test('%(playlist_autonumber)s', '02')
         test('%(autonumber)s', '00001')
         test('%(autonumber+2)03d', '005', autonumber_start=3)

yt_dlp/YoutubeDL.py

@@ -1271,21 +1271,20 @@ class YoutubeDL:
                 return outer_mobj.group(0)
             key = outer_mobj.group('key')
             mobj = re.match(INTERNAL_FORMAT_RE, key)
-            initial_field = mobj.group('fields') if mobj else ''
+            value, replacement, default, last_field = None, None, na, ''
-            value, replacement, default = None, None, na
             while mobj:
                 mobj = mobj.groupdict()
                 default = mobj['default'] if mobj['default'] is not None else default
                 value = get_value(mobj)
-                replacement = mobj['replacement']
+                last_field, replacement = mobj['fields'], mobj['replacement']
                 if value is None and mobj['alternate']:
                     mobj = re.match(INTERNAL_FORMAT_RE, mobj['remaining'][1:])
                 else:
                     break
 
             fmt = outer_mobj.group('format')
-            if fmt == 's' and value is not None and key in field_size_compat_map.keys():
+            if fmt == 's' and value is not None and last_field in field_size_compat_map.keys():
-                fmt = f'0{field_size_compat_map[key]:d}d'
+                fmt = f'0{field_size_compat_map[last_field]:d}d'
 
             if None not in (value, replacement):
                 try:
@@ -1322,7 +1321,7 @@ class YoutubeDL:
                 value = format_decimal_suffix(value, f'%{num_fmt}f%s' if num_fmt else '%d%s',
                                               factor=1024 if '#' in flags else 1000)
             elif fmt[-1] == 'S':  # filename sanitization
-                value, fmt = filename_sanitizer(initial_field, value, restricted='#' in flags), str_fmt
+                value, fmt = filename_sanitizer(last_field, value, restricted='#' in flags), str_fmt
             elif fmt[-1] == 'c':
                 if value:
                     value = str(value)[0]
@@ -1341,7 +1340,7 @@ class YoutubeDL:
                 elif fmt[-1] == 'a':
                     value, fmt = ascii(value), str_fmt
                 if fmt[-1] in 'csra':
-                    value = sanitizer(initial_field, value)
+                    value = sanitizer(last_field, value)
 
             key = '%s\0%s' % (key.replace('%', '%\0'), outer_mobj.group('format'))
             TMPL_DICT[key] = value