RIGHT after using auto-contrast, I checked Levels,
which now shows some areas that are mildly black.

Photoshop will make the grays go into deeper black
for eye-relief and because in real life there tend to be
real darks. SOMEtimes you may not want that of course.

After contrasting action, there will always be shown
some detail dropped, relative to the original image
still in memory, when you go dark-gray to black, and
bright-grey to white and that's what the white streaks
indicate, in this already downsized photo.

In large originals when you do this (the way to go),
you won't see such a striking difference in lost detail
between the two images current in memory, when
comparing action-results.