After converting a stock Eclipse project (having just an src/ source folder) to Maven (Configure...>Convert to Maven), the java and resource directories are set to src.

When configuring persistence via forge, a new persistence.xml is generated under src/main/resources. Turns out MavenResourceFacet has hard-coded paths for main and test resources : https://github.com/forge/core/blob/c178b19bbae655b04648be21e2a368a5e57cf7d3/project-model-maven/src/main/java/org/jboss/forge/maven/facets/MavenResourceFacet.java#L48

The resource paths should be inferred from the Maven pom instead, (<resources> and <testResources> nodes)