I have my Share button set to take shots immediately when I press the button (I have 280 shots of Second Son, I couldn't possibly use the press and hold style), but I've found every time I try and access the Share menu by holding the button it also takes a screenshot. Useful in a way, but sometimes I want to upload multiple screenshots from the past and I end up taking multiple useless screenshots of nothing trying to do this. For now I'm just screenshotting the PS4 home screen (so the shots go to a separate folder) but I'd really love some way to get directly to the share menu without a shot taken.
If you're using the screenshot config for the Share button, then a long press will take you to the Share menu.
I use this same config and sometimes mine will take an extra screenshot or two when trying to get in there.
I'm not sure if it's just some jitter on the button itself, or if the firmware is flaky when it comes to interpreting the Share button press, or whatever else could be wrong with it, but the behaviour you describe lines up with my own experience.