diff --git a/init.lua b/init.lua
index ffbf22e..7d00aa6 100644
--- a/init.lua
+++ b/init.lua
@@ -687,8 +687,10 @@ function M.load_textures(asset_aliases)
         }
     end
 
-    -- Update m.tile_size from first atlas (assume uniform; first atlas wins)
-    if m.atlases[1] then
+    -- Update m.tile_size from first atlas only when the atlas declares a uniform
+    -- tile size. Variable-size atlases (tile_size_px = null) keep the tile_size
+    -- that was set by the stub or tilemap JSON at load time (typically 32).
+    if m.atlases[1] and m.atlases[1].tile_size_px ~= nil then
         m.tile_size = m.atlases[1].tile_size_px
     end
 end