Hi, timhodkin.
What you're seeing is the result of the auto save function that saves open items every x minutes to prevent data loss should the application crash or the computer shut down unexpectedly. Each time auto save saves the item Outlook is sending an update because the task has changed and I'm sure it was sent with the option to "Keep an updated copy of this task on my task list" checked. The behavior is by design. Outlook can't tell that the task has changed as a result of the auto save. All it knows is that the task has changed and the task owner wants their copy of the task kept up to date.
I see two workarounds.
1. Instead of sending attachments send a link to a file. Opening a link shouldn't trigger the auto update and there's no need to leave the task open while working on the file.
2. Save the attachment to disk, close the task, then work on the file. When you're done with it remove the original attachment and replace it with the modified version.