Here's what I experience. I usually start with the flower nutes the last watering in veg. The defoliate a bit the last week of veg. Then switch to the 12/12 but the stretch really uses up some mineral nutes, calcium, magnesium and potassium.
In the past I've always struggled with pH at this point but that was not a problem this time, new type soil was and It showed a deficiency in calcium and potassium after the stretch. Using ProMix BX and i like it.
I have an auto too that when it when into stretch (20/4 light cycle) showed the same, with the yellowing top leaves but also had really hollow stems. I started adding more Ca on top of the soil using both CAN and CaCl @ 1300 ppm though spaced out a week. It's growling like crazy and one plant filling up the tent. But the stems are much stronger, still a little yellow on top. The auto is in FF Ocean Forest with added limestone and perlite.