The theme thinks the TextInput is using StageTextTextEditor, so it is setting the fontFamily property. However, you've asked the TextInput to use TextFieldTextEditor, which doesn't have a fontFamily property. Feathers is a little more strict now because people were trying to set properties that didn't exist on components and getting confused. This had a side effect of affecting people who were trying to change text renderers or text editors. It's not difficult to fix the issue, though. You have a couple of options.
First, you could simply tell the TextInput not to use the theme:
inputText.styleProvider = null;
However, this will also remove the background skin and other styling properties that you may want to keep.
What you probably want to do instead is create a new style name for TextInput. Something like this:
Then, you can create a new function in your theme to style the TextInput differently than the default. You might call it setCustomTextInputStyles(). Basically, copy the function that sets the default TextInput styles, remove any properties that it tries to set on StageTextTextEditor, and then tell it to use TextFieldTextEditor instead.
For MetalWorksMobileTheme, you'd want to remove this part:
>input.textEditorProperties.fontFamily = "Helvetica";
input.textEditorProperties.fontSize = this.regularFontSize;
input.textEditorProperties.color = LIGHT_TEXT_COLOR;
input.textEditorProperties.disabledColor = DISABLED_TEXT_COLOR;
Replace that section with your code where you set it up to use TextFieldTextEditor.
Then, in an override of initializeStyleProviders(), you can associate the custom style name with your function like this:
override protected function initializeStyleProviders();
this.getStyleProviderForClass( TextInput )
.setFunctionForStyleName( "custom-text-input", this.setCustomTextInputStyles );
For more details about extending a theme and creating a custom style name, see Extending Feathers Themes.