FYI you can look at Backup file size issue. There are plenty of tests there (and linked from it) to look at.
People only look so far (so no cause or workaround is known). It needs work by someone who sees it.
48234496 is 2E00000 in hexadecimal, so it fits the pattern of suspected filesystem or SMB issues.
I don’t know if anyone has ever tried testing with different SMB servers. It’d be interesting to know if a.Windows system as destination would have the same issue, or if Synology config is relevant to it.
It’d also be nice to see via Process Monitor (see other posts) if Duplicati wrote whole file. If so, what happened on what system to make the length wrong? Or is the length mis-reporting actual file size?
One of the linked GitHub issues found a persistent misreporting even when asking from PowerShell.