This is discussed in a recent topic here (though the initial question was about removing the icon from the lock screen).
I don’t think this has anything to do with available memory as I sometimes see the problem and I have plenty of memory both internal and SD card. Seems to be a bug so don’t hesitate to make Support aware of the problem to add weight to getting it fixed.
As a work-around when you’re doing a lot of photography, if you have registered a fingerprint correctly you should be able to access the camera pretty quickly, especially if you leave it open when you lock the phone.