With our testing(I work in IT), we were able to simulate the issue everytime in the PowerPoint windows application.
If we open YouTube and select a video signed in with a premium acct,
No ads, then if you copy and paste that exact link and embed it into the PowerPoint Preso, ads play.
If you then close the PowerPoint Preso down and open it portal.office.com web portal PowerPoint, the video will play without ads again.
We did the same with our enterprise YouTube acct (via Google). We sign in and can watch an approved video without issue.
Embed that same video into PowerPoint windows desktop app and you get ‘this video is restricted’, which only comes up when you are not signed into YouTube with your authorised acct, or video not approved.
If you then close that Preso down and open same Preso in. PowerPoint online it works as it should, even if nothing changes in between.
We’ve tried multiple ways of adding with no other way than to house PowerPoint online.
We are using this as our work around, while we try to convince MSoft this is actually an issues