This project works as expected on breadboard, the "final product" version, it's broken... After checking everything, multimeter, swapping circuit parts, etc... I concluded it was the ATtiny.
The code on the ATtiny is extremely simple and only uses 2 pins other than V+/GND... pin A = input, pin B = output. A inputs by a button (edge), which "permanently" flips B output high or low.
On the breadboard version, works great, button causes output pin to stay high/low. On the "final" one - it acts as a momentary switch basically, let go of button and B output pin goes low...
I accidentally inserted an ATtiny wrong before, and the thing basically started smoking... after that I figured it was fried - and it behaved the same way that I described above (as a momentary switch)... I took a new ATtiny and programmed it, tested on breadboard, and soldered into the project and got the above results.
Given that the one in my soldered project is behaving like the one I burnt, my question is: could I have damaged \ fried the ATtiny when soldering it?
Not knowing what your are soldering with and to I can only assume nothing too extreme. I will say damaging an IC by soldering it with the iron on the device too long is not easy to do. I've abused a number of IC's with no issues.
So.... Perhaps ESD?
Else you can post the schematic and photo of hardware.
I had a problem similar problem as i soldered my fist prototype with USB and MicroSD (ESP32S2) the MicroSD just did not work. After I removed the MicroSD connector from my old circit and tested it, it still worked.
Because I could not find the problem I just solder a new one and it worked fine. (I unsolder the old one too and used more wires and it worked after that too).
So I would either try to unsolder and solder your solder joint /bad solder joints) or just try to create a new one. (That only works if your circuit has no problems)