key backup/cross signing bootstrap sometimes generates incompatible restore key

I can't reproduce this every time, but got it so far three times (one on matrix.org).

You setup key backup in element-ios and choose backup key.
Then you share the key with some other app (to save it).

When everything is fine the share dialogue closes, you can tap on continue and the key can be restored just fine.

When the failure happens the share dialogue will still be displayed and element-ios doesn't recognize it as 'saved'. So you click on 'copy' and can afterwards tap on continue.
When you now try to restore the key in element-web it'll say 'Wrong Recovery Key'.

I've sent a rageshake.

Strangely the same recovery key will work in element-ios.
I've sent an additional rageshake after restoring the key in element-ios in case it helps.

Start by reviewing the two reported rageshakes and tracing the key-backup setup, share, copy, continue, and restore flows in element-ios. Reproduce the intermittent path if possible, then verify that a recovery key saved or copied from iOS restores successfully in both element-ios and element-web.

Written by the indexing model from the issue text.