Well at least it was one for me.

After the setup on the breadboard, I tested the reading of the time. All worked fine as long as USB was connected. There is an example sketch in the arduino ide, which sets the time of the build. I could read time and display it, but the moment I was running from battery, I git a "not RTC source found".

As I'm not an electrical engineer at all (I try to expand my elementary school battery-switch-lamp knowledge as I'm going) I figured it our "backwards". I compared the adafruit DS3231 board with the PiHat and noted that the resistors were different, which is easy to understand, once you realized, that the PiHat is made for 5v logic, while the adafruit one is made for the 3.3V logic (at least I hope, I have right in my head now).

Anyways here is the reworked PiHat, with the correct resistors: (and 2 cables to connect a new coincell, because the soldered in was already drained)