What Storage Type? If it’s your machine, you can maybe even name remote software.
What is transport to the destination? Presumably some sort of network? How reliable?
Check your job log, Complete log value for RetryAttempts which handle upload fail.
For details, set advanced options log-file=<path> and log-file-log-level=retry.
For what I think this is, type this in Google and read AI Overview about temporary files:
"transfer" "part extension"
Duplicati’s name ends at .aes. Anything after that, like .part, was done by transport.
As for different results, are all settings the same, including Remote volume size which
could be seen in the size of the large dblock files. Are the daily dblock often smaller?
EDIT:
What is the Remote volume size? If transport is unreliable, big files are more likely to fail.
Another factor might be upload volume. Duplicati defaults to 4 uploads at the same time,
which might stress a network. asynchronous-concurrent-upload-limit can adjust this.