assertj: Illegal reflective access warnings with SoftAssertions and Java 9+

Using AssertJ 3.9.0 and JUnit 5 with Maven Surefire 2.19, I get the classic Java 9 warning:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.assertj.core.internal.cglib.core.ReflectUtils$1 (file:.../org/assertj/assertj-core/3.9.0/assertj-core-3.9.0.jar) to method java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain)
WARNING: Please consider reporting this to the maintainers of org.assertj.core.internal.cglib.core.ReflectUtils$1
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

When I use Assertions instead of SoftAssertions, the warning doesn’t show.

My environment:

$ java -version
openjdk version "9.0.1"
OpenJDK Runtime Environment (build 9.0.1+11)
OpenJDK 64-Bit Server VM (build 9.0.1+11, mixed mode)
$ uname -a
Linux sagan 4.15.0-0.rc8.git0.1.fc28.x86_64 #1 SMP Mon Jan 15 17:04:36 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

I have a suspicion that this is somehow related to either JUnit 5 or Maven Surefire 2.19. When I used JUnit 4 with Maven Surefire 2.20, AssertJ threw no warnings.