Desugaring failed for jar library

I am working on this project: https://github.com/muxinc/mux-stats-sdk-exoplayer/tree/min_sdk_version_fix It is Android app called demo that have dependency to a module from a same project called MuxExoPlayer which have a dependency to a MuxCore.jar library.

When I try to compile the project with minSdkVersion 16, I get desugaring issue on jar file, error:

Transform artifact full.jar (project :MuxExoPlayer) with DexingWithClasspathTransform
AGPBI: {"kind":"error","text":"Default method desugaring of `com.mux.stats.sdk.muxstats.MuxStatsExoPlayer` failed because it's hierarchy is incomplete. The class `com.mux.stats.sdk.core.events.EventBus` is missing and it is the declared super class of `com.mux.stats.sdk.muxstats.MuxBaseExoPlayer`","sources":[{}],"tool":"D8"}

When I compile with minSdkVersion 24, I do not get this error and project compile just fine.

MuxCore.jar is also built with gradle, here is the gradle.build:

apply plugin: 'java-library'

// In this section you declare where to find the dependencies of your project
repositories {
    // Use jcenter for resolving your dependencies.
    // You can declare any Maven/Ivy/file repository here.
    jcenter()
}
tasks.withType(JavaCompile) {
    sourceCompatibility = '1.7'
    targetCompatibility = '1.7'
}

dependencies {
    // This dependency is exported to consumers, that is to say found on their compile classpath.
    api 'org.apache.commons:commons-math3:3.6.1'

    // This dependency is used internally, and not exposed to consumers on their own compile classpath.
    implementation 'com.google.guava:guava:23.0'
    implementation 'org.json:json:20180130'

    // Use JUnit test framework
    testImplementation 'junit:junit:4.4'
    testImplementation 'org.mockito:mockito-core:2.23.4'
}

What am I missing here ? How do I need to compile MuxCore in order to compile with android minSdkVersion 16 ?