Opacity changes randomly?

I ran into a strange issue recently, that the opacity of different elements in a file change in a seemingly randow order. I have this file with a couple of process illustrations, mainly arrows and boxes with text, and all is working fine and I close the document. The next time I open it things have unexpectedly changed, so that objects that were fully visible suddenly have opcacity 32%. It is aslo haphazardly happening with objects within groups, so some parts of the group can have full opacity and some are 32%.

I have gone in and manually adjusted them back to 100% and continued working, but then the same thing happened when I closed and reopened the file again later. 

One of the first things I tested then was to copy the relevant parts of the illustrations to a new document in case the old was corrupt in any way, but the same problem happened again in the new document. 

I have never seen this problem before in the 20+ years I have been using Illustrator, so if someone knows what is happening and how to avoid it I would be grateful.