I have an Adsense ad unit running on every page of my personal blog. This morning while looking at doubleclick setup options- I noticed the ad unit type was set to text only. When I tried repeatedly to reset the ad unit to both text and image ads-- the option would not hold. Have you run into this issue? PS-- the ad unit has been running since 2013 and both image and text ads have been in evidence since that time. Only lately, have I only noticed just text. I am the only authorized user for my account.
I have never face issue like this. You can try to create a new ad unit and check the issue persist or not.