[outtmpl] Fix replacement for `playlist_index`

test/test_YoutubeDL.py

@@ -687,6 +687,7 @@ class TestYoutubeDL(unittest.TestCase):
         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

@@ -1300,16 +1300,16 @@ class YoutubeDL:
                 else:
                     break
 
-            fmt = outer_mobj.group('format')
-            if fmt == 's' and value is not None and last_field in field_size_compat_map.keys():
-                fmt = f'0{field_size_compat_map[last_field]:d}d'
-
             if None not in (value, replacement):
                 try:
                     value = replacement_formatter.format(replacement, value)
                 except ValueError:
                     value, default = None, na
 
+            fmt = outer_mobj.group('format')
+            if fmt == 's' and last_field in field_size_compat_map.keys() and isinstance(value, int):
+                fmt = f'0{field_size_compat_map[last_field]:d}d'
+
             flags = outer_mobj.group('conversion') or ''
             str_fmt = f'{fmt[:-1]}s'
             if value is None: