I think I may have found it. The atlas-mvn script overrides the MAVEN_OPTS setting forcing a smaller memory footprint.
To test this theory can you instead of executing /opt/atlassian-plugin-sdk-6.2.14/bin/atlas-mvn clean package .... execute /opt/atlassian-plugin-sdk-6.2.14/apache-maven-3.5.4/bin/mvn clean package ...
If you want to keep using the atlas-mvn script then you can either opt to export your memory settings at ATLAS_OPTS and try again. This may not work having multiple memory flags set, but you can also comment out line 64 export MAVEN_OPTS="-Xms512M -Xmx768M $ATLAS_OPTS" of the atlas-mvn script and keep your original MAVEN_OPTS.