greyish leaftips and dieing branches after topping

topping doesn't cause that unless you gave it some disease from your shears. More likely it is coincidentally occurring. do you foliar spray? stop doing so if you do. foliar sprays without a good reason or in reaction to a problem are an e-peen that cause more problems than any potenetial benefit provided. 17-19C is quite cold for this plant. It's biological processes may be struggling. Could also be behind the discoloration - the dark blue or purple If pH were low, some of the first symptoms you'd see are calcium deficiency, which i don't see here. Looks like N is a tad high, but only the cauase of the slightly too lush/glossy leaves and not the primary damage seen. Any changes to fertilization lately?