Download IDs are not managed by our software. They are managed by Joomla which calls them Download Keys. They are stored in the #__update_sites table which stores the location of the update sites of each extensions and can be managed in System, Update, Update Sites.
When you click on Rebuild, Joomla will create the update site records in the #__update_sites table afresh. This will lose the Download Keys already entered. I have reported this publicly is as a bug over a year ago and had also reported this privately since the Rebuild button was added. Beyond that there's nothing else I can do except tell you to not use the Rebuild button on that page.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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