Hi obviously,
Thank you posting your question using Apple Support Communities. We understand that you're experiencing issues with your videos uploading to iCloud, and we're happy to assist.
First, you mentioned that you've created a backup via iCloud after noticing this. It is important to know that photos and videos that appear on the iCloud website would have been uploaded using iCloud Photos. When using iCloud Photos, your library is stored separate from your backups.
With that in mind, let's ensure that iCloud Photos is enabled on your device. Set up and use iCloud Photos
If you've confirmed that this setting is enabled, let's check the status of the upload. You can check that by going to Settings > [your name] > iCloud > Photos, or by opening the Photos app, going to the Photos tab, and scrolling to the bottom of your screen.
We'd also recommend checking to ensure that you have enough iCloud storage available. Manage your iCloud storage
Let us know what you find.
Regards.