If you run into a situation where your app is not honouring the app-extensions settings in your DALP file, check to make sure that the DALP is a plaintext file.
Specifically, be careful of text editors that emit UTF-8 signature or Byte Order Mark (BOM) prefixes to the file contents. Such signatures will interfere with how the DALP file is processed, leading to some of the contents not being parsed. A UTF-8 file with a BOM starts with the 3 bytes 0xEF 0xBB 0xBF, the UTF-8 encoding of the U+FEFF character.
Such files, once part of your application package, will interfere with the ability of your app to retrieve the app-extension values from the XML at runtime.
The solution is to remove the BOM such that the DALP file is plaintext, using any reasonable encoding. How to remove the signature bytes will depend on the editor you are using. See this StackExchange Q&A that discusses how to check if a file starts with a BOM.
A good overview is described here: Display problems caused by the UTF-8 BOM (w3.org)