From bug #257752 comment 2: "The activity notification is now masked for a while after it is triggered. That is basically what KDE3 konsole does. The mask interval is intended to avoid the flooding of notification and is hardcoded as 15 seconds at the moment. Maybe that interval should be user configurable." When using activity monitoring while developing javascript with devel tool like webpack the hardcoded period of 15 seconds is too long which makes subsequent edits to source file (within this period) to lack notification. Configurable period when notifications are masked would solve this problem. Reproducible: Always
There are a lot of new notifications settings since this report. As well as a way to set threshold limit in minutes. Reopen if none of these still don't work for you.