Maven can't see sibling dependencies

I've just spent another 2 hours of my life due to this Maven bug: Project x with two modules a and b. b depends on a:

x
├── a
│   ├── pom.xml
│   └── src
│       └── main
│           └── java
│               └── net
│                   └── skybert
│                       └── A.java
├── b
│   ├── pom.xml
│   └── src
│       └── main
│           └── java
│               └── net
│                   └── skybert
│                       └── B.java
└── pom.xml

B.java makes use of A.java:

package net.skybert;
import net.skybert.A;
public class B {
}

Now, whenever I want to build b, I must run mvn clean in b before compiling it, or else b will not be able to find a's artifact (even though the a artifact is in ~/.m2/repostiory). Pretty amazing, right?

The work around is to turn off incremental compilation:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.7.0</version>
  <configuration>
    <useIncrementalCompilation>false</useIncrementalCompilation>
  </configuration>
</plugin>

With this addition to the project's root POM, I can run mvn compile in b and don't get compile errors because net.skybert.B can't find net.skybert.A.

Posting this here to 1) help others 2) get it out of my system and 3) give my future self a laugh when pondering the same mystery 2 years from now.