When you want to select text in a text box that includes the starting character, if you drag from right to left, if you drag past the beginning, the selection can't be deleted with the keyboard
I am not sure that I understand what you do.
I selected a field in the tag panel (in which the whole text got selected when I clicked into it).
So I clicked in the white area of the text box so that the blue selection disappeared.
Now I press the left mouse button and drag the cursor from right to left across the field and even further. Result: the text is selected.
With such a selection I undertook several actions:
I pressed delete and the selected text was deleted (as expected).
I pressed the cursor-right button and the selection vanishes, the text cursor stands behind the right-most character (as expected).
I pressed the cursor-right button plus shift and the selection gets reduced by one character, the very first one (as expected).
So what did you do and where did you get a reaction that you did not expect?
(Uh, yes: I did the same with an editable field in the files list with the same results)