When the backup first starts, Duplicati scans the filesystem to find files that should be backed up and it also notes the total size of all those files.
If some files grow larger after they are scanned at the start of the backup - but before Duplicati has backed them up - then it can throw off this ‘bytes to go’ calculation.
It is superficial and can be ignored. The UI should probably be adjusted to hide the ‘xx bytes’ portion if it goes negative.
A fix was made a few months ago for another situation where this number could be negative: files were temporarily counted twice against the total bytes processed. But your version has that fix so this is a different issue - probably file growth during backup.