Hi,
I've never seen that.
Looking at your screenshot, it seems that you've customized the email.
Go in the menu System>Emails and edit the email. Normally, the quantity is displayed by the tag {LINEVAR:PRODUCT_QUANTITY} in the HTML version.
Do you still have it ?
Did you change the line where that tag is ?
The default is:
<td style="border-bottom:1px solid #ddd;padding-bottom:3px;text-align:right">{LINEVAR:PRODUCT_QUANTITY}</td>
Also, this tag is filled with the line:
'PRODUCT_QUANTITY' => $item->order_product_quantity,
in the preload of the email. Do you still have that code in the preload section ?
These are the things I would recommend checking first.