Odometer is resetting to a lower mileage

Yes, I am cross posting but I am at a complete loss on this one.

I just recently bought a 1998 Safari (Astro's sister van from GMC) and had driven it for about a month and I broke 69000 miles, or at least I thought I did a day or so later I noticed it said 689xx miles. I thought I just misread earlier so I watched this time and so I found that whenever my odometer hits 69033 miles it reverts back to 68969 miles.

Now this happens every single time I hit 69033 miles. I have asked everyone I know and not a single person has a clue. Now I have the check engine soon light on, but normally I can get this to go out if I crank the gas cap on really tightly. I also have a scan tool on the way so I can check the diag codes from the computer.

So anyone have a clue why or how this could be happening. Also, could someone tell me how this digital odometer works? Is it a sensor that reports to the computer and the computer displays the mileage on the odometer? Or is there other components that I am not aware of in the system?