UPDATE JUNE 2020: THIS WORKS AGAIN https://addtoprofile.linkedin.com/#header2
I have had an answer from Linkedin support on this subject : "Please also note, with the new site redesign, the Certification information will not auto-populate for members. Clicking on the button will simply take the member to their Certifications section of their Profile where they'll need to manually enter the information.". I have then asked if this is temporary or final and was answered "At this time, we're not sure. However, feedback like yours is definitely important and has the potential to influence product changes."
So I guess that the best thing is to write to Linkedin to share that it's an issue : https://www.linkedin.com/help/linkedin/forum . That the place where I got my answer from