So mine went down to like 3GB available last night, now at 6GB available. I think it's a bug and will be corrected. Believe this happened a few years ago and was fixed by beta 3.
The way Apple does storage with APFS is weird also. That 3-6GB on a 64GB device is probably the true value left. I've had apps tell me I have only 3-4 GB left but Apple says 30GB. It's the way they have there stuff work. Like even on Mac, the about my Mac will say one value but some apps like Forklift &/or Daisydisk will say another. Some apps corrected this to show what About My Mac was showing but it seems Big Sur messed it up. So like on Mac, About My Mac shows I have 436GB left available but both Forklift & Daisydisk show 404GB left available. But it will be fixed....