This is likely due to the update a while ago - which effectively made it so Diplomacy doesn't work like it used to. (I believe the technicals were that with Diplomacy researched you USED to not get revolts unless you were below 67% - going off of memory, mind you) Now diplomacy gives more resources overall with the old effect no longer working.
Forced loyalty does not stop revolts, it just stops loyalty from going any lower than it already is. If you are below 80%, it is not advisable to use this.