Spanner testsuite used EmebddedEngine for running tests. EmebddedEngine supported running only a single task and if ti was asked to run multiple tasks, it silently ignored it and run just one task. After migration of the testsuite to async engine (DBZ-8381) which supports running multiple tasks, the testsuite started to fail.
Following tests are failing:

[ERROR] Failures: 
[ERROR]   BasicSanityCheckIT.shouldStreamUpdatesToKafka:104 
Expecting actual not to be null
[ERROR]   DataTypesIT.shouldStreamUpdatesToKafkaWithTheCorrectType:100
[ERROR]   GracefulRestartIT.checkUpdatesStreamedToKafka:60 
Expecting actual not to be null
[ERROR]   KafkaTopicPartitionIT.checkRecordsWithSameKeyAreInSamePartition:70 
Expecting actual not to be null
[ERROR]   PgDataTypesIT.shouldStreamUpdatesToKafkaWithTheCorrectType:104
[ERROR] Errors: 
[ERROR]   LowWatermarkRecordIT.shouldStreamUpdatesToKafka:67 NullPointer Cannot invoke "java.util.List.stream()" because "records" is null
[INFO] 
[ERROR] Tests run: 9, Failures: 5, Errors: 1, Skipped: 0

All the tests fail because the tasks actually don't start streaming changes, but it's not very clear what the root cause is. It may be issue with registering JMX bean, but I'm not sure about it as there is lots of reballances and other stuff, so there may be also some other issue.

The strange thing is that tests start passing when tasks.max is specified. And event when the number of tasks is 10, this is the default value of tasks.max.