To me, 'till' suggests a point in time i.e. until , and when you want to mention two things happened at different points in time in the past, you ought to use 'had'.
Clock struck 12 - first event
reading continued till 12
I was reading till 12.
v/s
I had been reading till clock struck 12 - continuous state of reading till 12.
I know, the first one seems natural and grammatical; However, I don't think the second one is wrong either.
For the past few days, my mind hasn't been working properly, and it seems that I've forgotten all the basic principles of english. I'm sure what has happened to me

I work till 12 everyday. work all the time until 12; how does it represent continuity ?
I worked till 12 last night. how does it represent continuity ?