There should be no power interruption as the 300w PSU is powering 2 arduinos, a bme280, a 16x2 display and the stepper motor.
Everything else is 240vac, and powered independently.
However, everything is possible, so I will have a look to see if I can see any issues there.
If it's missing steps, it's missing a lot in a short period, and giving up the goat mid way through a move cycle, there's 80 full turns between my physical limits, I'm using approximately 70 turns, and a stepper motor that due to gearing has 10200 steps per rev.