I think this issue is the way the Pinterest link is set up (or code) not a version issue.
I agree. I don't think the Pinterest code within Juicebox-Pro has changed since it was first introduced.
I have found the following format works perfectly, is there a way to "inject" this?
It might technically be possible to override the default functionality of the Juicebox-Pro Pinterest Button but it would take a lot of work (to extract the required information, build up the custom share URL and use it instead of the one Juicebox-Pro currently uses) and such a modification would not be officially supported.
Using your own example, I notice that if you are not already logged in to Pinterest when you click the button, you will be presented with a 'Join Pinterest' window.
If you click the 'Log In' button on this page, then you will then be redirected to the 'Log In' page where you need to click 'Log In' again.
The share URL that Juicebox-Pro uses skips the 'Join Pinterest' page meaning that a user who is not already logged in will have to click 'Log In' only once.
(On the 'Join Pinterest' page, if you enter your existing Pinterest details and click 'Sign Up', this actually logs you in and goes straight to the share page but this seems counter-intuitive, especially as there is a 'Log In' button on the 'Join Pinterest' page which doesn't actually log you in, even though your existing Pinterest details have been entered on the page, but just redirects to a 'Log In' page.)
In any case, I have notified the developers. Thank you for reporting the issue.