Do not update palette for L mode GIF frame

Tests/test_file_gif.py

@@ -100,6 +100,18 @@ def test_l_mode_after_rgb() -> None:
         assert im.mode == "RGB"
 
 
+def test_l_mode_transparency_after_rgb() -> None:
+    with Image.open("Tests/images/no_palette_with_transparency_after_rgb.gif") as im:
+        expected = im.convert("RGB")
+        d = ImageDraw.Draw(expected)
+        d.rectangle([(0, 0), (64, 128)], fill="#000")
+
+        im.seek(1)
+        assert im.mode == "RGB"
+
+        assert_image_equal(im, expected)
+
+
 def test_palette_not_needed_for_second_frame() -> None:
     with Image.open("Tests/images/palette_not_needed_for_second_frame.gif") as im:
         im.seek(1)

src/PIL/GifImagePlugin.py

@@ -477,8 +477,11 @@ def load_end(self) -> None:
             self._prev_im = expanded_im
             assert self._prev_im is not None
         if self._frame_transparency is not None:
-            self.im.putpalettealpha(self._frame_transparency, 0)
-            frame_im = self.im.convert("RGBA")
+            if self.mode == "L":
+                frame_im = self.im.convert_transparent("LA", self._frame_transparency)
+            else:
+                self.im.putpalettealpha(self._frame_transparency, 0)
+                frame_im = self.im.convert("RGBA")
         else:
             frame_im = self.im.convert("RGB")
 
@@ -487,7 +490,7 @@ def load_end(self) -> None:
 
         self.im = self._prev_im
         self._mode = self.im.mode
-        if frame_im.mode == "RGBA":
+        if frame_im.mode in ("LA", "RGBA"):
             self.im.paste(frame_im, self.dispose_extent, frame_im)
         else:
             self.im.paste(frame_im, self.dispose_extent)