If the name of your world file contains any of the following brackets: [ { } ], and you have a resources.zip resource pack packaged with the map, the map will not load that resource pack when opened.

I have provided an example world file in the attachments. The resource pack packaged with this world file is an empty one to demonstrate that the game is not loading any resources. This bug will occur even if the resource pack is not empty. When you open the map, view the launcher and you will notice that there is no indication of any resource pack being loaded. If you remove the brackets in the world file name and load it, you will notice that the launcher DOES indicate a pack being loaded this time.

Code analysis by __null in this comment.