Hi Mitch… it’s been a recurring issue, here’s the most recent post on it: Modules disappear from Module Manager when updating VASSAL
If you propose to work with the Vassal source code, you can find some getting started developer guidance on the documentation pages.
I have had the issue myself (on Mac too) and logged it on GitHub a while back. It still happens but it might have a different cause. I wonder - as per @Brent_Easton’s comment - if it’s associated with a cloud-related file.