Armstrong's first (left) footprint was already erased by..himself!
The first footstep was right next to the LM footpad, where the astronauts stepped many times, still hooked to the LM by a rope.
Those footsteps were done in the shadow (the ladder was on the LM frontside where there was shadow)....They also stepped there to climb back.
For me, there is no doubt the first footstep was within seconds stepped upon by Neil and Buzz, thus gone forever.