How can I restore notes and tabs without iCloud backup on iPhone 12?

No one can tell you what happened. Have you checked your recently deleted folder in Notes and Photos? Not backing up due to lack of space is something you could have and should have addressed. While Apple gives you 5GB of free iCloud Storage, you could have increased that to 50GB at the incredibly low cost of $1 a month/$12 a year. Chances are you spend more money on other items in your life you may not even need. But making sure your data is backed up, so you don't lose it, seems kind of important to me. Maybe I'm just different that way.

You say you have enough space on your phone? Open Settings > General > iPhone Storage and see how much space you actually have available. If you didn't have enough space in the cloud to backup and you may be low in space on your phone, you'll lose data. It's like hoping to pour 2 gallons of water into a 1 gallon bucket.