In the first movie, Doc explains the time circuits. There’s a display for where you’re going, where you’ve been, and where you are.
After old Biff yoinked the DeLorean and brought it back, the display for “where you’ve been” reads November 12, 1955.
It's a simple gross oversight on behalf of Doc. It wasn't until after Marty and he arrived in 1985a that Doc realized Biff stole the Delorean and picked up the almanac -- top of Biff's cane etc.
The previous "BEEN": November 12, 1955, was replaced with "October 26, 1985" once they arrived in 1985a.
I'm presuming the time machine didn't store data more than what was displaced on the panel. The Timer in 'SLIDERS' initially had a similar issue.
They just didn't.
They didn't know that Biff had stolen the time machine until they returned to 1985. So, there was no reason to assume anything was amiss. Doc had no reason to be paying close attention to the date at the bottom of the screen, because as far as he was aware, the Delorean had just been parked the whole time when they were in Hilldale. Marty had no real reason to notice as the passenger either.
Doc just put the date in, they returned to 1985, noticed it was different, then started trying to figure out what had gone on.