I've just tried again on two devices running Android 14, with the same results:
When I click on "Delete" in Vivaldi's download manager immediately, the file is deleted (not just removed from the list of downloads).
When I move the file and then click on "Delete" in Vivaldi, the file is deleted too.
When I rename the file, move it and then click on "Delete", the file is deleted too.
When I copy the file and then click on "Delete", the original file in the Downloads folder is deleted and the copy isn't.
In the desktop version if Vivaldi, we have the option to clear the list (with the sweep icon). It would be really helpful to have such an option in the mobile version too.