My reported version is 5.14.0-1 (debian package) which doesn't seem available in the list of versions here. I have copied some files from some CDs and they had permissions: -r-------- which is not what my umask would give them. I then had to change the permissions to allow writing. Reproducible: Always Steps to Reproduce: 1. Copy some files from a CD to home directory
umask doesn't matter because these aren't new files, they are the result of a copy. Basically KIO emulates `cp -a`, i.e. preserves permissions. cp has command line options to choose whether to preserve or not, drag-n-drop doesn't ;) So it's tricky to guess what the user wants. In your case you obviously didn't want readonly files, but in general, preserving permissions when copying is seen as a feature rather than a bug. So I'm not sure how to "magically make it right for everyone in all cases".
Maybe it is possible to find out if the source file system is mounted "read-only", resulting in all files to have the "read-only" attribute.