Time Machine making full backups instead of incremental after macOS Tahoe 26.1 upgrade

I am experiencing the same problem - even changing the back up frequency to once per day Time Machine still appears to make a full back up each period.

I was having this same issue, and I found a solution that fixed it for me.

Step 1: run this command in the terminal "sudo tmutil disable". This command turns off Time Machine.

Step 2: run this command in the terminal:

sudo xattr -d com.apple.backupd.BackupMachineAddress "/System/Volumes/Data" 2>/dev/null || true

sudo xattr -d com.apple.backupd.ComputerName "/System/Volumes/Data" 2>/dev/null || true

sudo xattr -d com.apple.backupd.HostUUID "/System/Volumes/Data" 2>/dev/null || true

sudo xattr -d com.apple.backupd.ModelID "/System/Volumes/Data" 2>/dev/null || true

sudo xattr -d com.apple.timemachine.private.structure.metadata "/System/Volumes/Data" 2>/dev/null || true

This command deletes the attributes that TimeMachine uses to track Time Machine backups and determine whether a new full backup is needed. I believe that one of these is being messed up in Tahoe, which causes Time Machine to only do full backups. FWIW, I think running Disk Utility is what messed mine up.

Step 3: Reboot computer

Step 4: run this command in the terminal "sudo tmutil enable". This command reenable Time Machine.

Step 5: Rerun time machine, this first backup will be a full backup because we have reset the identifiers. The second backup should be incremental.

Good luck!