How can I get IDEA to retain 'Ajc' as the project compiler?

IDE uses the Maven configuration for compiler when you have Settings (Preferences on macOS) | Build, Execution, Deployment | Build Tools | Maven | Importing | Detect compiler automatically option enabled.

Please make sure to use latest IDE versions as the com.nickwongdev:aspectj-maven-plugin support has been added in later versions: https://youtrack.jetbrains.com/issue/IDEA-230356

if problem remains can you provide a sample project that would show the Maven configuration you use?

Disabling "detect compiler automatically" works. Thanks! But the plugin didn't help, I have it configured as such:
   

<pluginManagement>
    <plugins>
        <plugin>
            <groupId>com.nickwongdev</groupId>
            <artifactId>aspectj-maven-plugin</artifactId>
            <version>1.12.6</version>
            <configuration>
                <complianceLevel>11</complianceLevel>
                <source>11</source>
                <target>11</target>
            </configuration>
            <executions>
                <execution>
                    <goals>
                        <goal>compile</goal>
                        <goal>test-compile</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</pluginManagement>
<plugins>
    <plugin>
        <groupId>com.nickwongdev</groupId>
        <artifactId>aspectj-maven-plugin</artifactId>
    </plugin>
</plugins>

Are you sure the aspectj compiler is effectively used for the project when you compile from command line?

Try explicitly specifying the compiler using compilerId configuration properly of maven-compiler-plugin. Since the default compiler is always javac. The aspectj value will set it to aspectj compiler.