IntelliJ target bytecode reverting

You need to specify the source and target versions in your pom.xml file, but since the maven-compiler-plugin is added by default, an easier way to do that would be to set the following properties:

<properties>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
</properties>

FYI.

I still had this problem despite setting the pom.xml with both the settings above from the answers as mentioned by @isapir and @carlos-a-ibarra respectively.

I discovered this was due to an Intellij setting:

This config can be found in Build,Execution,Deployment > Build Tools > Maven > Importing

By default, it is set to the 'Internal JRE' setting, which in the latest version of Idea is 11. I had to select it to use the 1.8 JDK the project was configured to use.

This is annoying, because if you have a mix of 11 and 8 projects, you'll have to manually toggle this setting back and forth.

It caused the module output to always fall back to 11 every time the pom.xml was reimported.

It's a really annoying bug in IntelliJ.

You need to dd this to your POM:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.3</version>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
            </configuration>
        </plugin>
    </plugins>
</build>