MIC5891 driver output voltage
Look at this table in the data sheet: -

It basically tells you that with a 100 mA load current, the chip may &quot;lose&quot; some 2.3 volts in its output transistor thus lowering a 12 volt power supply to about 9.7 volts on the output.
Of course that's the maximum value of &quot;lost voltage&quot; so if you see 10 volts at 115 mA then that doesn't really surprise me: -

When I put even a small load on the output (115ma measured), (it can
handle 500ma) the voltage drops to 10VDC, but the input supply voltage
still stays at 12V.

The good thing is that you double checked that the supply hadn't changed in voltage.