Build out of order

I'm using Sencha Cmd 5.1.1.39 and Sencha Touch 2.4.1.

When I do "sencha app build testing", the resulting app.js has certain class files out of order, so that class X is used by class Y, but Y is included before X. As a result, the app doesn't work.

When I run the app in development mode, I don't get any warnings about files being included dynamically. And the class Y has X listed in its "requires" section.

How can I figure out how to fix this? The same build works with Sencha Cmd 4.0.2.67.

Build out of order