Why is my dog's back wound leaking pus a month after spay surgery?

Based on the photos you provided, I would highly suspect that this is from a burn (temperature or chemical). If your veterinarian used heating discs, a heating blanket or a heating table during surgery, the temperature may have been too high and could have lead to these burns developing. I've seen this happen at my clinic where a technician used a blow drier to heat up a patient because the temperature was too low on the dog and the dog developed severe burns. I would treat them as burns (antibiotic therapy, pain medications and keeping the area clean) and wait for the scabs to fall off and the tissue to heal. I would seek a second opinion from a dermatologist as well in regards to this and whether or not Laly should be on immunosuppressant drugs for this as it may be a condition in which these are not needed. Again, the location, pattern and appearance have me suspicious of a burn. I hope this helps! Please let me know if you have any other questions or concerns and I would be more than happy to help you further.