Spring boot application stop serving traffic while Kafka consumer rebalancing
Boot 2.1 is end of life. The last release, last month, was 2.1.18. The current 2.2.x release of spring-kafka is 2.2.14.
If you can upgrade to (at least) Boot 2.2.11 (spring-kafka 2.4.11 - Boot brings in 2.3.x by default) (and a broker >= 2.3), you could consider configuring incremental cooperative rebalancing.
Current releases are Boot 2.4.0 and spring-kafka 2.6.3.
https://www.confluent.io/blog/incremental-cooperative-rebalancing-in-kafka/