Cbbackup estimation message error

Hi,
I have a problem with cbbackup after node add.

Backup work correctly but I have estimation message that not work and at the end not correspond the % of completion.

Version: Community Edition 5.1.1 build 5723

Example:
[################## ] 88.7% (4579641/estimated 5161292 msgs)
bucket: chat, msgs transferred…
: total | last | per sec
byte : 2303678450 | 2303678450 | 6189536.4
done

msgs in the bucket: 4579641 CORRECT
estimated: 5161292 WRONG

Thanks,
Jacopo