Progress bar stays at 99.9% for a long time before finishing

I am using duplicacy in combination with a wasabin cloud storage.

When I run the backup, at first the progress bar slowly fills up to 99.9% (this is what I read from the css style attribute).
After that, the progress bar stays at 99.9% while the system is still visibly busy (as in, they slow down other operations

Later in the log I see that UPLOAD_FILE operations were happening in the time while the progress bar was at 99%.

So… could this be a bug/flaw in the progress bar calculation?