Password protection - Numbers

I have a Numbers file that requires a p/w to open.

I recently saved it on iCloud, since I want to be able to access it on my iOS devices.

When I open it on my iPad, it prompts me for the p/w, which is good.

But if I make any changes to the file, iCloud saves the changes, which it should, but the p/w protection is removed.

Am I doing something wrong, or is this a bug with iCloud syncing?