I increased partitions for one existing topic (2->10), but was surprised to
see that it entirely reset the committed offsets of my consumer group.

All topics & partitions were reset to the earliest offset available, and
the consumer read everything again.

Documentation doesn't mention anything like this. Is this how it's supposed
to work, or a bug?

I would've expected the consumer offsets to not decrease at all, especially
for the topics that I didn't even touch.

For the altered topic I would've expected that consuming the previously
existing partitions 0 and 1 would've continued from the position where they
were, and naturally starting to read the new added partitions from 0.

I added partitions according to the "Modifying topics" section of Kafka
0.10.0 Documentation:

"To add partitions you can do

 > bin/kafka-topics.sh --zookeeper $ZOOKEEPER_HOST --alter --topic
altered_topic --partitions 10
"

Previously this topic had 2 partitions.

For the consumer I'm using:
kafka.javaapi.consumer.ConsumerConnector.createMessageStreamsByFilter()

And version is:

<groupId>org.apache.kafka</groupId>
<artifactId>kafka_2.11</artifactId>
<version>0.10.0.1</version>

Kafka cluster itself is kafka_2.11-0.10.0.1.

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