This bug was originally reported at AdoptOpenJDK here and has all the details:
https://github.com/AdoptOpenJDK/openjdk-build/issues/1309

There appears to be a regression from JDK 8 in the annotation processing engine related to modules. The reproducer works on OpenJDK 8u212-b10, but fails with JDK 11.0.4+11 and better (including JDK head) with -proc:only.

Steps to reproduce:
$ git clone https://github.com/jerboaa/JDKCompilerBug.git bug
$ cd bug
$ mvn clean package
$ java -showversion -cp target/jdkcompiler-1.0-SNAPSHOT.jar com.bilalekrem.jdkcompiler.Main true
openjdk version "14-internal" 2020-03-17
OpenJDK Runtime Environment (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head)
OpenJDK 64-Bit Server VM (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head, mixed mode, sharing)
java.lang.AssertionError
at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46)
at jdk.compiler/com.sun.tools.javac.comp.Modules.enter(Modules.java:247)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.readSourceFile(JavaCompiler.java:836)
at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$ImplicitCompleter.complete(JavacProcessingEnvironment.java:1576)
at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:657)
at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1356)
at jdk.compiler/com.sun.tools.javac.code.Symbol.apiComplete(Symbol.java:663)
at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.getKind(Symbol.java:1424)
at com.bilalekrem.jdkcompiler.Main.getKind(Main.java:85)
at com.bilalekrem.jdkcompiler.Main.main(Main.java:58)
Printing element kind
Multiple: -- resulted: true
Single: -- resulted: false
Exception in thread "main" java.lang.RuntimeException: Test failed!
at com.bilalekrem.jdkcompiler.Main.main(Main.java:63)

Actual:

java.lang.AssertionError
at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46)
at jdk.compiler/com.sun.tools.javac.comp.Modules.enter(Modules.java:247)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.readSourceFile(JavaCompiler.java:836)
at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment$ImplicitCompleter.complete(JavacProcessingEnvironment.java:1576)
at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:657)
at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1356)
at jdk.compiler/com.sun.tools.javac.code.Symbol.apiComplete(Symbol.java:663)
at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.getKind(Symbol.java:1424)
at com.bilalekrem.jdkcompiler.Main.getKind(Main.java:85)
at com.bilalekrem.jdkcompiler.Main.main(Main.java:58)
Printing element kind
Multiple: -- resulted: true
Single: -- resulted: false
Exception in thread "main" java.lang.RuntimeException: Test failed!
at com.bilalekrem.jdkcompiler.Main.main(Main.java:63)

Expected (with JDK 8):
$ java -showversion -cp target/jdkcompiler-1.0-SNAPSHOT.jar com.bilalekrem.jdkcompiler.Main true
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (build 1.8.0_222-b10)
OpenJDK 64-Bit Server VM (build 25.222-b10, mixed mode)

Printing element kind
Multiple: -- resulted: true
Single: -- resulted: true
Test passed!

Note that passing "false" or no-arg to the reproducer, in which case -proc:only isn't set, the reproducer passes with jdk head:

java -showversion -cp target/jdkcompiler-1.0-SNAPSHOT.jar com.bilalekrem.jdkcompiler.Main false
openjdk version "14-internal" 2020-03-17
OpenJDK Runtime Environment (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head)
OpenJDK 64-Bit Server VM (fastdebug build 14-internal+0-adhoc.sgehwolf.openjdk-head, mixed mode, sharing)
Printing element kind
Multiple: -- resulted: true
Single: -- resulted: true
Test passed!