Looks like adding UIBackgroundModes to the Xcode iOS exporter Custom Plist doesn’t merge well with Audio Background Capability / Bluetooth MIDI Background capability and you end up with duplicate keys in the plist.
Ie:
Custom Plist in Projucer (With Capabilities turned off):
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist>
<dict>
<key>UIBackgroundModes</key>
<array>
<string>external-accessory</string>
</array>
</dict>
</plist>
Generates this Info-App.plist:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist>
<dict>
<key>UIBackgroundModes</key>
<array>
<string>external-accessory</string>
</array>
....
<key>UIBackgroundModes</key>
<array/>
</dict>
</plist>