EEPROM only retaining a value for a short duration
In your comments you ask &quot;if a write endurance issue can explain this specific type of EEPROM failure mode.&quot; From my past experience I would say the answer is absolutely yes.

We have identified a situation though where we could perform a lot of writes/erases to that location in production if some steps are not performed correctly. The endurance is a million writes and we could be hitting that.

As you may know, the endurance spec of a EEPROM only applies to normal usage. If the device is written rapid-fire (for example a firmware bug causing the device to get stuck in a loop performing writes immediately one after the other) than the endurance will be much shorter. It sounds like that may be happening here.

Previously in my career I've always seen write endurance failures look like sticking bits that seem to never take on a new value. Could this &quot;decay&quot; phenomenon that I am seeing also be a explained by excessive writes?

Yes. While completely &quot;burnt&quot; (i.e. fatigued) EEPROM cells will be stuck at a single value, it is also entirely possible for EEPROM fatigue to cause the memory operation to just degrade, rather than fail completely.

Footnote / war story illustrating this phenomenon:
I was on a team where we built a device with EEPROM memory storage. The customer complained that the EEPROM was failing to hold its value. They sent it back to us, we tested it and it worked fine. We sent it back to them and it failed again. This whole loop happened one more time until we visited the customer on site and found the real problem. The basic root cause:

Customer was operating the device in a manner which caused the EEPROM to erase over and over in rapid succession, fatiguing the part. This was a surprise to us, another case where &quot;no customer would ever do it that way&quot; turned out to be a faulty assumption.
Every time we tested the product at our facility we operated it &quot;normally&quot;, so we did not see the problem.
Here's the key: luckily we had device-level components engineers on our team, and one of those engineers informed us that EEPROM memory cells can have a self-healing effect over time. If you let the device rest, it will actually start to operate somewhat normally again, but obviously that device should no longer be trusted. (Note this was very surprising to me and I still don't understand the physics behind it, but all my empirical observations tell me this engineer was correct.)

So the reason this problem was so infuriatingly difficult to troubleshoot is that the EEPROM cells got fatigued by the customer to the point of failure, but then they had a chance to rest during their time being shipped from the customer facility to ours, so they worked fine in our testing! Then we returned them to the customer, where they would promptly get fatigued again and fail again.