According to the JP leaks, didn’t the film originally start with the Luke and Leia flashback, and then transition to present day Leia training Rey? If Kylo Ren re-forged their Force bond during her training montage, then it presumably would have taken place before Kylo met Palpatine.

I guess you could say Vader’s helmet and other Force-related objects can concentrate power to achieve more difficult tasks? Or Palpatine helped Kylo re-establish that connection through Vader’s helmet? I’m not really sure, that is a good question.