Unfortunately, some daylilies are unlikely to do well when grown at high temperatures. A researcher found that 85F and 95F continuously were too high for good growth of a daylily, 'Purity'. Many of the flowers were blasted at 85F and no plant grown at 95F flowered. Plants at both temperatures grew poorly.
When days are very hot (for daylilies) how well they survive and flower may depend on how cool the nights are, for how long the days are hot, on how deep their roots are, how well they are able to use water, etc.
To grow Stella successfully in hot summer southern gardens may require trying different spots and growing conditions in your location.
