You are experiencing a problem due to a known bug in the 21.05 versions that has since been fixed. See “Fixed a proxy may used instead of the original when dropping from file manager to Timeline” in Shotcut - New Version 21.06.29. However, an upgrade will not fix your broken project. You need to turn off the proxy mode and use the Replace action on the bad timeline clips to fix it. Or, if it it a small project maybe start over. Sorry for the problem, but this is one reason why we switched the download page to the older version 21.03.21 as our “Recommended Stable Version” but you can also choose to upgrade to the latest through the news/blog posts. Most people have found the latest version 21.06.29 to be very stable and fix most bugs introduced in version 21.05.