@Impossible_PhD Questionable.
Hillary wasn't Clinton to differentiate her from Bill. You can't do that with Bush, since they were both George [Herbert] Walker Bush. I've mostly heard him referred to as "Baby Bush," which IMO is less respectful. In any case, HC wanted to be distanced from BC for obvious reasons, while GWB wanted voters to view him as part of the Bush political dynasty.
Harris is only a day into being the presumptive nominee, so I expect things to change in the coming months. I often call them by their full names for a long time before eventually switching to last name only, and it hasn't been a very long time. Then there's Palin, who (and I might have just not been paying attention) I have never heard referred to as "Sarah."
And some intentionally use their first names to be relatable, like Eisenhower (Ike, although not technically his first name) or Sanders (Bernie, although never a Presidential nominee)
Also, two female candidates are not enough to spot a pattern.