This message was deleted.

This is unfortunately a known issue: https://github.com/gradle/gradle/issues/12388 (with a workaround, fortunately!) (see also https://github.com/gradle/gradle/issues/22271 for your use case)

Thank you, Thomas – you saved my time with that. 🙂 I’ll go with:

distributionFile.convention(
    signPluginTaskProvider
        .flatMap { signPluginTask ->
            when (signPluginTask.didWork) {
                true -> signPluginTask.outputArchiveFile
                else -> project.provider { fallbackValue }
            }
        }
)