Files in the IPFS network will disappear after the network decides the file is not popular enough and no longer needed. This is about 1 day after upload.
To prevent this DTube pays 25% of your earnings to ipfsstore.it. They charge 0.044 USD/Gigabyte per month. After this money is depleted they delete your video. So your video should at least stay online till payout after 7 days. To be sure they stay online you could seed the videos yourself.

I tried to pin and seed your videos from my ipfs node, but I can't fetch the videos over IPFS. Seems like they are really offline.