Maven multi-module non-resolvable parent - maven

Question

i'm facing a build error in maven, which i'm unable to explain, my project layout is:
root
|--parent
|--project
where root (aggregator) and project both use "parent" as their parent.
i used this layout in another project, which maven is able to build.
however, in this project, if i'm trying to build, i get a failure on non-resolvable parent pom:
[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[FATAL] Non-resolvable parent POM for maven.test:maven-test-project:9.9.9-SNAPSHOT: Could not find artifact maven.test:maven-test-parent:pom:9.9.9-SNAPSHOT and 'parent.relativePath' points at wrong local POM # line 11, column 10
if i remove the parent node from the root (aggregator) pom, maven is able to build the project successfully.
if i use an older version of maven (3.3 instead of 3.6) the project is built successfully as well.
here are the poms for all 3 projects:
root
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>maven.test</groupId>
<artifactId>maven-test</artifactId>
<version>9.9.9-SNAPSHOT</version>
<packaging>pom</packaging>
<name>Maven Test</name>
<parent>
<groupId>maven.test</groupId>
<artifactId>maven-test-parent</artifactId>
<version>9.9.9-SNAPSHOT</version>
<relativePath>parent/pom.xml</relativePath>
</parent>
<modules>
<module>parent</module>
<module>project</module>
</modules>
</project>
parent
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>maven.test</groupId>
<artifactId>maven-test-parent</artifactId>
<version>9.9.9-SNAPSHOT</version>
<packaging>pom</packaging>
<name>Maven Parent</name>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<java.version>1.8</java.version>
<maven.compiler.source>${java.version}</maven.compiler.source>
<maven.compiler.target>${java.version}</maven.compiler.target>
</properties>
</project>
project
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<artifactId>maven-test-project</artifactId>
<version>9.9.9-SNAPSHOT</version>
<name>Maven Project</name>
<parent>
<groupId>maven.test</groupId>
<artifactId>maven-test-parent</artifactId>
<version>9.9.9-SNAPSHOT</version>
<relativePath>../parent/pom.xml</relativePath>
</parent>
</project>

Answer 1

There's a conception issue.
"root" cannot be at the same time a parent and child of the same Maven module "parent".