I have heard a lot of people having problems getting a smooth transition dissolve or opacity change. The last 10% of your dissolve seems to abruptly end.
Well until adobe fixes this problem i have a workaround that will make it smooth.
the clip you want to dissolve you make a key frame on the opacity and make it 100% and then where you want it to end you make a key frame and set it to a low opacity like 10% or less
so you can barely see it, ( depending on how dark or light your clip is) then you make a third keyframe about 6-9 frames down the timeline to bring it down to 0%.
This will make a nice smooth fade out at the end.
Play with the settings but that third key frame is key to extend that last little bit of fade a few frames longer.
i sometimes will even extend that keyframe about 10 frames further down the timeline for a super smooth dissolve.
Hope this helps!