It sounds like maybe one of the programmable function buttons is set up to change to illustration mode. Then it would just take an accidental pressing of this button to change rather than having to walk through the menu to make the change.
That's what I was thinking. I must have it on a programmable function and accidentally hit that button. I will have to read up on that because honestly, I don't use those functions and don't know how to set it up or change. :(
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