In my similar situation (Not exactly the same because the ST showing was under an Air Canada flight, not QR), This is the reasoning I was given from the QRSupport page on twitter:

"Upon checking, allow us to clarify that your booking is already confirmed and the message "ST" will get removed automatically once Air Canada confirms your seat selection." They Also told me that "ST" means a seat in reference to the correspondent request in Air Canada sector.

In the end, it does seem that the "ST" code under special request, does directly refer to seating selection/confirmation and as said, not much concern needed.