Mazda 3 2010 P0791 After Transmission Flush - Temperature-Related Shift Issues

I own a 2010 Mazda 3 with a 2.5L automatic transmission. Recently, I had my transmission flushed just before reaching 150,000 miles (I now realize this was not recommended). After the flush, my vehicle began displaying Diagnostic Trouble Code P0791. Initially, the car would not shift out of first gear. Upon investigation, I discovered that the garage had left the transmission dipstick out of its proper position during the service. Once I reinstalled it correctly, the vehicle ran normally, but both the engine and transmission warning lights remained on. I attempted to let the codes self-clear by driving the car for several months. However, over time, the shifting became increasingly intermittent—especially after prolonged driving—and recently, the car completely failed to move forward and stalled at a stop. The issue appears to worsen as outside temperatures rise, with symptoms becoming more noticeable when ambient temperatures reach 100°F in Tennessee. I have already replaced both the input and output shaft speed sensors. Despite these repairs, the P0791 code persists and the shifting behavior remains problematic.