Are you using PCG Volumes and running a PCG graph to generate your foliage, or are you using the Procedural Foliage Volume?
The Procedural Foliage Volume gets compiled out of builds, so can’t possibly be re-generating foliage in a packaged build.
Doing a quick scan through the source, it looks like the PCG Component and PCGGraph can run in a packaged build, so if you’re using those, it does seem most likely that your PCG is being re-run on startup. If you’re happy with what it’s given you in-editor, you can disconnect the generated instances from the PCG and either delete the PCG Volume or move it into a WP runtime layer that you never load.