I am encountering this with a java_export that contains io.netty:netty-tcnative-boringssl-static. I can provide a repo example if it helps.

To reproduce, create a java_export with any/all of the following artifacts:

"@maven//:io_netty_netty_tcnative_boringssl_static_linux_aarch_64",
"@maven//:io_netty_netty_tcnative_boringssl_static_linux_x86_64",
"@maven//:io_netty_netty_tcnative_boringssl_static_osx_aarch_64",
"@maven//:io_netty_netty_tcnative_boringssl_static_osx_x86_64",
"@maven//:io_netty_netty_tcnative_boringssl_static_windows_x86_64",

The generated pom.xml contains the group, artifact, scope, and version, but is missing the classifier. What I would expect:

<dependency>
  <artifactId>netty-tcnative-boringssl-static</artifactId>
  <classifier>linux-aarch_64</classifier>
  <groupId>io.netty</groupId>
  <scope>runtime</scope>
  <version>2.0.61.Final</version>
</dependency>

The maven_coordinates tag does contain the classifier, but the unpack_coordinates function does not account for it: https://github.com/bazelbuild/rules_jvm_external/blob/master/private/rules/maven_utils.bzl#L7

maven_coordinates=io.netty:netty-tcnative-boringssl-static:jar:linux-aarch_64:2.0.61.Final

I'd be happy to put up a PR with a bit of guidance. The maven coordinate string in the javadoc linked above seems to follow a different syntax than what is present in the tag.
groupId:artifactId[:type[:scope]]:version while the tag looks to be groupId:artifactId[:type[:classifier]]:version without a scope.