Rather annoying little bug since upgrading to plasma 5. When selecting a file / folder dolphin used to keep the selected file / folder highlighted when either it lost focus and then gained focus again or when working within dolphin you entered the directory selected and pressed back from the toolbar. This made it easier to work with files. At the moment you have to re-select the file / folder to work with it. As an example, if i had a folder called "temp" in dolphin i could double click it to open it and when i pressed back to go to the root folder with "temp" in it the folder "temp" would still be highlighted so if i wanted to delete it i could just press DEL and happy days. Now, when i press back after double clicking it, i have to click the "temp" folder once to highlight it and then press DEL. It may sound like a little thing but when you are manipulating files / folders en masse during the day it is a pain
Sorry to say, but this was an intentional change (https://invent.kde.org/system/dolphin/-/commit/122fee5625f0285ec4ebda79162c72390989eb2a). I made sure that users can only delete files by pressing the Delete key if they explicitly selected the file beforehand. This change makes sure that users are less likely to act on files by accident, which can lead to data loss. Others have also mentioned that they dislike the extra selection they now have to do in those very specific scenarios like the one you mentioned. We are considering other ways to give feedback when files are being moved to the trash e.g. a sound effect, an animation, or a visible message in the status bar, but none of those are currently implemented, so if a user presses Delete without noticing, they are currently still likely to not notice that they have moved a file to the trash. The current Dolphin behaviour you are reporting here makes it at least less likely they have a file selected in the first place.
I think it would be easier to make it an option in dolphin even if it defaults to not being selected. It makes plasma very "clunky" in this usage scenario