Can you share your Project Level Gradle ?
You can exclude this way also

configurations {
    compile.exclude group: "junit", module: "junit"
}

I have a TestFixture module that contains shared Test dependencies for different modules in my application. However, this module is completely unnecessary in my release build as my application (outside of testing) never references any of the items in it.

Is there a way to exclude this TestFixture module from my release build?

All my modules

testImplementation project(':testfixtures')

TestFixture module

implementation project(':app')
implementation project(':server')
implementation project (':data')
implementation project(':cache')
implementation project(':core')

implementation "androidx.test.ext:truth:$androidxExtTruthVer"
implementation "com.google.truth:truth:$truthVer"
// Testing Moshi
implementation "com.squareup.moshi:moshi-kotlin:$moshi_version"
implementation "com.squareup.moshi:moshi:$moshi_version"
kapt "com.squareup.moshi:moshi-kotlin-codegen:$moshi_version"

... Other implementations