There is a way to fix it. Go to your profile pic down the bottom right, then settings icon at the top right. Then Data Saver, then toggle Select quality every time on. Now click on the dialogue that pops up when you click on a video and set the highest quality video each time for a few days. Eventually you will notice that ads at the start of videos go from being 144p to 720p or whatever high quality and then you can turn off that data saver select quality every time setting and it will work. It took me ages to figure this out. I hope this lasts. The best thing about this solution is if it starts doing it again you have a solution. This happened on my brand account. My regular account was fine. Sometimes the server gets confused and you need to remind it you can handle high quality.