My now favorite example for what you describe is this blog post "I no longer understand my PhD dissertation". However, it's worse, even the same person experiences this - because over time we are different people.

To quote from the blog post:

> I was curious to see how much of the dissertation I can still grasp, five years after the fact. I figured it couldn’t hurt my ego if I refreshed my mind with past mathematical glories.

> How wrong I was.

> This was not the casual read I had in mind. The notation was alien. I even had to scour the examiner’s report to direct me to the key results. And while I could have sworn this was a well-written thesis, I repeatedly found myself bamboozled by my own prompts. “The result now follows easily…” may have made sense back when, but now the author-turned-confused reader can profess that it most certainly does not follow easily, at least in his own mind.

https://medium.com/@fjmubeen/ai-no-longer-understand-my-phd-...