Text selection with shift and arrow keys only expands selection, not contract

I've come across a frustrating issue with Indesign versions 22/23/24 on a Mac that doesn't usually occur.
Confirmed normal functionality on another mac, and a PC.

In short, expected behaviour is:

Select text box

hold shift key, press either arrow key to select text, and press the opposite arrow key to deselect text whilst holding shift key down, release and rehold shift key to select/deselect in opposite direction.

likewise if up arrow key pressed, select line, down arrow key, deselect line and so on.

Observed behaviour is:

Select text box

hold shift key, press either arrow key to select text, when opposite arrow key is pressed, it extend the selection in the appropriate direction.

I'm sure this is an otherwise undocumented keyboard shortcut I've inadvertantly used but I cannot for the life of me recall what it is.

Thanks in advance for any assistance!

Dave