RE: adding an album cover when none is visible in the cover modal, i.e. it is blank —
What is the expected default behaviour upon executing the shortcut "Ctrl+Alt+A"? Should the Windows default file manager open the relevant album folder?
What do you consider to be the relevant folder?
Without a specific folder name for pictures, the folder of the currently selected file(s) gets opened.
See also the Options>Directories
You still have not said which folder you consider "relevant" for a particular file.
MP3tag cannot guess where a picture for a file may be hidden.
The intended behaviour to add covers is described in the documentation:
Thank you. I am applying %_parent_directory%, but for some reason, it does not apply consistently to the applicable track directory/folder. I am now testing and considering other options — as per said documentation.
Sidenote: Could the regular cleanup of the Mp3tag Library perhaps have any bearing on this event?
It first tries to load the folder that's configured as Cover art directory at Options ▸ Directories.
If this is empty or can't be resolved to a valid folder, it tries to find a file with one of the supported file extensions (jpg, jpeg, png, or webp) in the selected file's directory.
If no file is found, it opens the current working directory.