The jni functions in GoogleARCoreAndroidHelper.cpp are not correctly exported, when we tried to build a Test project it failed with this exception:
java.lang.UnsatisfiedLinkError: No implementation found for void com.google.arcore.unreal.GoogleARCoreJavaHelper.onApplicationCreated() (tried Java_com_google_arcore_unreal_GoogleARCoreJavaHelper_onApplicationCreated and Java_com_google_arcore_unreal_GoogleARCoreJavaHelper_onApplicationCreated__)
Adding JNI_METHOD in front of them fixed the issue on our branch. This is an easy fix, might be good for 4.21
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-66052 in the post.