Absolut weird.... the same for me... I did not change anything (no update/no extra reboot/etc)
Now:
INFO: root.pxar: backup was done incrementally, reused 1.91 GiB (97.3%)
INFO: root.pxar: backup was done incrementally, reused 70.79 GiB (99.8%)
INFO: root.pxar: backup was done incrementally...