Why does the youtube offline videos get expired

Weird behavior of offline videos. I had downloaded a lot videos from office for offline viewing at home. But when i went home they were showing expired and when i clicked on them it showed connect to internet to update. So i went online but even then it was showing connect to update, but after long hours of searching over the internet i couldn't find any solutions. So i first removed them from offline and re-downloaded them.

That was okay, but the same thing happened today hence i am frustrated. My questions are like this: 1)Why are they getting expired 2)Solution for the above question.

Screenshot:

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