Gradle platform with java-test-fixtures

Question

I am configuring dependency versions for an multiproject gradle build in a centralized way. This way it works:

// root build.gradle.kts -- test configuration centralized
subprojects {

    apply(plugin = "java-library")

    repositories {
        jcenter()
    }

    val junitVersion = "5.5.2"

    dependencies {
        "testImplementation"(platform("org.junit:junit-bom:$junitVersion"))
    }
}

This way does not. While configuration phase is OK:

// root build.gradle.kts -- test && test fixtures configuration centralized
subprojects {

    apply(plugin = "java-library")

    repositories {
        jcenter()
    }

    val junitVersion = "5.5.2"

    dependencies {
        "testImplementation"(platform("org.junit:junit-bom:$junitVersion"))
    }

    if (convention.findPlugin(JavaTestFixturesPlugin::class.java) != null) {
        dependencies {
            "testFixturesApi"(platform("org.junit:junit-bom:$junitVersion"))
        }
    }
}

...yet compileTestFixturesKotlin task fails. And the error is:

Could not find org.junit.jupiter:junit-jupiter-api:.

What's wrong with java-test-fixtures plugin? Or with my code maybe?