I had this exact same problem with Jessie on an HP ProBook 650. The bug
was in X.org and was fixed when I upgraded to Stretch. The problem, as
I recall, was that certain display hardware inverts the sense of the
intensity, so that instead of it going from 0 (dark) to max (bright) it
goes from 0 (bright) to max (dark). X.org sets the display brightness
to what it thinks is max on the log in screen to compensate for the
possibility that was turned all the way down by a previous user.
What I found as a "workaround" was that I could log in as normal and
then by using the brightness control function keys I could increase the
display brightness after logging in.
Roberto C. Sánchez