As scheduled jobs can run without your doing anything, did you also see that Duplicati was idle?
If there is a chance that some backup started, About → Show log → Live → Profiling might help.
A reproducible case where mono gets that busy with little Duplicati help would be interesting, and potentially a workaround could be found or the cause narrowed, but mono is kind of stagnant now.
Battle plan for migrating to .Net8 is underway, to move off of mono and stagnant .NET Framework.
Please run mono --version to see how up to date it is. Mono releases has generic release notes. Maybe there are OS limitations too. I don’t have a Mac, so can only provide generic Duplicati help.
Likely not real memory. How To Use Activity Monitor: Master Class mentions that and other kinds. Perhaps you can check your menus (if they are like picture there) to see what it is, and get others.