I didn't really know the answer to that but I found this online: https://www.huffpost.com/entry/why-reheated-coffee-tastes-ba...

I do know that while I love espresso I don't have a particularly "discerning palate" (many of the lighter roasts that seem to be so popular among fancy coffee shops taste far too sour/acidic for my liking), but even I can taste the grossness of coffee that has been sitting on a hot plate too long. It just tastes really "burnt", bitter, and acrid.