If I have an automatic dispenser set to sweep only and then select an item to be swept with the highest priority (!!), dupes will not react in any way.
Normally a !! task will cause dupes to drop everything to complete it ASAP. The only way to get dupes to immediately complete the sweep task is to set the automatic dispenser itself to !! priority.
Even if the automatic dispenser is set to priority 9, dupes will still do other tasks first.
This is very annoying and doesn't fall in line with other !! priority behaviour. In cases like slime, where fast sweeping is very important, you can't simply !! sweep an area - you have to sweep it AND then set the automatic dispenser to !! priority. One would think a !! sweep would be enough...
A very inefficient solution would be to have the ore dropper always on !! priority, but then you constantly have alarms and a yellow border.
P.S. it doesn't matter if dupes are on standard priorities or have ++ priority for tidying and storing - they will still choose to do other tasks first before completing a !! sweep...
Have an automatic dispenser (or perhaps normal storage container) set to "sweep only" and set to priority 9. Sweep something using !! priority. Watch as dupes run other supply/construction/dig/research jobs and literally run past the sweep job without even reacting.