I am confused, you have a stepper motor that as far as I can see is rated at 12V and you are driving it with 5V. Datasheet I found is here
You do realise the starting current inrush of inductive loads is many times the rated current especially with gearing after the motor. You do have diodes to battery supply on all pins to stop back EMF when the step is complete probably at higher rating thans what is in a ULN280x series chip.
You doi realise that if you need several Amps of current at 12V you need more
Amps if you drop the voltage to 5V.
You do realise that the ULN280x series are rated at 500mA (0.5A) on ONE
pin driving multiple pins at once means the TOTAL current in the GND pin (sum of the pins 'on') must be less than 1A to 1.5A depending on manufacturer and many other factors.
Without seeing your FULL circuit as a diagram it will be difficult to determine what you are trying to do.
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