What might the reasons be that:

tasks.withType(Test).configureEach {
  useJUnitPlatform()
}

cause Gradle to ignore the

--tests

command line flag and seemingly run the entire test suite? Whereas defining it at the per task level works correctly:

tasks.named('endToEndTest', Test) {
  useJUnitPlatform()
}

tasks.named('test', Test) {
  useJUnitPlatform()
}

There are some plugins applied that may be affecting things in some way.