Root Exception not shown in Web UI

Description
On the job details page in the Exceptions → Root Exception tab, exceptions that cause the job to restart are not displayed.
This is already a problem since 1.9.0 if jobmanager.execution.failover-strategy: region is configured,
which we do in the default flink-conf.yaml.

Workarounds
Users that run into this problem can set jobmanager.scheduler: legacy and unset jobmanager.execution.failover-strategy in their flink-conf.yaml

How to reproduce
In flink-conf.yaml set restart-strategy: fixed-delay so enable job restarts.

$ bin/start-cluster.sh
$ bin/flink run -d examples/streaming/TopSpeedWindowing.jar
$ bin/taskmanager.sh stop

Assert that no exception is displayed in the Web UI.

Expected behavior
The stacktrace of the exception should be displayed. Whether the exception should be also shown if only a partial region of the job failed is up for discussion.