It looks like you've found a bug in WPILibJ. I've looked at the source for the Relay class, and it appears that it only allocates a Resource object large enough track relay channels on the first DIO module.
Java version:
Code:
private static Resource relayChannels = new Resource(tDIO.kNumSystems * kRelayChannels);
This isn't enough, since a separate resource is allocated for Forward and Reverse.
C++ version:
Code:
Resource::CreateResourceObject(&relayChannels, tDIO::kNumSystems * kRelayChannels * 2);
Until it's fixed in an update, you could copy the Relay.java source code from ~/sunspotfrcsdk/lib/WPILibJ/src into your project and correct it by adding in the factor of 2.
If this fixes the issue a bug report should be submitted on FIRSTForge.