Full stack trace not working

I’m testing a custom plugin under v4.9. The runtime execution of the plugin is failing (expected) and the trace is cut short as normal. At this point there is no indication of the line in my code displayed in the trace with 30 more lines to display. Specifying the full stack trace is not showing the extended stack trace as expected.
This shows the normal stack trace:
./gradlew aSt -s
These options have been used to try to display the full stack trace:
./gradlew aSt -S
./gradlew aSt --full-stacktrace
The end of the stack trace always is displayed as:

Caused by: org.gradle.api.InvalidUserDataException: No value has been specified for property 'outputFileName'.
        at org.gradle.api.internal.tasks.TaskValidationContext$Severity$2.report(TaskValidationContext.java:52)
        ... 30 more

I got this as well. My stacktrace is always abbreviated (with ... X more), regardless of whether I use -s or -S. I use Gradle 6.0.1.