I suspect that it is because it enters a stretch that is underlain by limestone. Associated with that are some small springs, which may provide just enough cooling to allow the trout to hang on for a few more weeks than from the SGL down to Cedar Crest Blvd. Additional evidence for this is that in a couple of pools I have seen Golden RT survive the summer and winter in the past. The Game Preserve stretch is particularly bad for warm water temps due to the width and long stretches of shallow water there and above that stretch. If you like that general area, however, I would recommend fishing Mill Ck. It is a stream that is stocked by a co-op and has a small wild Brown population. One has to seek the pools, however, because it is shallow for long stretches between the pools.
The extremely low numbers of summer survivors and the warm water temps in general during heat waves suggest that the springs are quite limited in number and flow. Two of these smaller springs are visible and dry up each summer. The recent rains in the past 1.5 weeks were the saviors for the trout that I am catching because the stream had been very low, which allowed for rapid warm up and some trout mortality. I was not sure that many had survived the most recent heat waves until I fished. Yesterday was an educated guess that some probably made it and the water was probably cool enough for the fish to be active. At this time of the year on the Jordan one has to be opportunistic and watch for good conditions as short lived as they may be.
There is a much larger spring that enters behind Home Depot just below Rt 22 . Farther down in Allentown there is another spring and an accompanying half mile stretch with a fishable wild brown trout population and, of course, surviving stockies throughout the summer. That down town stretch saw very limited stocking for the past two springs, however, due to Gordon St bridge construction.
I would add that the upper mile of the stocked Jordan from Rt 309 down along Rt 100 may hold fish longer than in the SGL. That’s my guess due to the habitat and flows that I have seen, but I could be wrong. The stream is more constricted and possibly more shaded than in the SGL. Given that you fish the upper Jordan maybe you have fished that stretch and know that my speculation is wrong.
A few more timely rain events with moderate air temps following may allow some trout to survive until the trico hatch starts.
Hope this info helps. One good thing is that the stream certainly is not crowded at this time of the year!