Thanks @Max for solving this. I needed to put

<dependency>
    <groupId>org.mapstruct</groupId> 
    <artifactId>mapstruct-processor</artifactId> 
    <version>${mapstruct.version}</version> 
</dependency>

as a runtime dependency for some reason. previously i only imported it during the build step during annotation processing. subsequently i had to use spring-boot-mvn-plugin repackage goal to repackage the multiple modules to a standalone fat jar. this allowed the mapper classes to be able to import classes from other modules without errors