I don't think your PH is way off. Having 6.2 in flowering is oke, 6.3-6.4 would be perfect.
I don''t think is boron, in my opinion this is a (CA)Calcium deficiency last stage.
Strange because you have been feeding for a long time, you even gave some when you where in growing stage...
Maybe you have been overfeeding, therefore your medium(soil) has many nutrients in it and it has been into lockdown, therefore your plant doesn't get the nutrients it needs.
Just flush your plant a bit with water ph 6.2, catch up your runoff and if it holds more then 2 EC you have been overfeeding, if it holds less then 1.2 EC, you have been underfeeding.
Calculating is knowing :)
Goodluck growing :)