Mecabricks has been around for years and I did not hear about anything like that. I am not saying that you are lying, simply that there may be another explanation for your issue.

Like for any software you shall regularly save your work. This is the basic. This will prevent from loosing too much time in the event of an issue. Furthermore, Mecabricks retains all version of your work so you can go backward in case you have an issue further down the line.
Mecabricks also provides a "recover autosave" version. If your browser crashes, this feature located in the File menu will recover the model before the crash. This is only available when you first re-open the workshop. If you reload it multiple time it is of course lost. I am not a wizard.

I think there must be a valid explanation for this disappearing build issue and this could happen because of technical errors. This assignment writer could prevent you from losing too much time and it will help you to retain all versions of Mecabricks.