this is understandable; in the ELCD database, End of Life processes like waste treatment are all forced to have the product on the output side since this was a requirement of openLCA < 1.7. To preserve the mass balance, an input reference flow is moved to the output, with negative sign. I post an example here:
To use it, add the "waste incineration of plastic.." flow into the process that is actually creating waste to be incinerated, on the input side, and also with negative amount (so, if the process produces 5 kg of waste to be incinerated, add -5kg to the input side, "provided" by the wast incineration plant).
This approach is also used in the ecoinvent database.
Since openLCA 1.7, this is not necessary any more, and we will consider this in future databases, although the approach works of course still.