Note this issue is related to using keyboard shortcuts ONLY. Don't use the mouse.
Move the cursor to J10 and place a value in it (e.g. 10)
Copy with [Ctrl]+'C' then move cursor to M5 press [Enter] to 'Paste'
Hold down the [Shift] key and press the [Down Arrow] to extend the selection to include M5 & M6 - This is where the error occurs.
Instead of a the desired selection of [M5:M6] a selection of [A2:M5] is made.
The desired selection was to be followed by [Ctrl]+'D' to 'Fill down'.
This error is easily reproduced
Hi @Charlie, thanks for reporting
*** This bug has been marked as a duplicate of bug 47818 ***