It doesn't look like the spout is respecting maxUncommittedOffsets in all cases. If the underlying consumer returns more records in a call to poll() than maxUncommittedOffsets, they will all be added to waitingToEmit. Since poll may return up to 500 records by default (Kafka 0.10.1.1), this is pretty likely to happen with low maxUncommittedOffsets.
The spout only checks for tuples to retry if it decides to poll, and it only decides to poll if numUncommittedOffsets < maxUncommittedOffsets. Since maxUncommittedOffsets isn't being respected when retrieving or emitting records, numUncommittedOffsets can be much larger than maxUncommittedOffsets. If more than maxUncommittedOffsets messages fail, this can cause the spout to stop polling entirely.