My apologies, I was tired and didn’t look too closely when I thought I’d found the problem. You’re correct about that line: it’s unrelated to your problem.
I’m not sure what Is causing your problem, but my guess is that it may have to do with the vendors folder being copied during a deploy (see: Johnpbloch/wordpress moved to a new configuration and WP goes missing). IIRC Trellis does this to speed up deploys, but it may be that when it does, it prevents the “install” event from happening on your package because it is already “installed” from the perspective of Composer. You might be able to solve the issue by removing vendor from project_copy_folders.