Just received my Brewjacket here in Italy and I've brewed the first hefeweizen. Before doing that, I've tested the device filling my 15 liters stainless steel fermenter just with water to have an idea of cooling capacity.
I had easily achieved 64.5°F / 18°C with a 79°F / 26°C external temperature. After that, testing with fermenting wort in these latest days, I incurred in the same problem you're describing, and I believe it's a matter of how temperature probe is reading fermenter temperature (that is not 100% reliable if it is not installed into a thermowell). As soon as the system reads that set point temperature is achieved, it stops (after a delay that I think is embedded into the program), but my sensation is that this is somehow a "false reading" and that the real temperature is almost a degree higher. For this reason, the cooler starts again after few minutes. I think that these starts/stops are not good, so I've just set the temperature one degree higher and I'm monitoring what will happen. Since I want to ferment at 71.5°F / 22°C and now I'm at 73.5°F / 23°C, if this trick works, I'll set the system at 70°F / 21°C and when I'll be there, I'll set up again the system at 71.5°F / 22°C.
Keep you posted about it.