Does keeping espresso in a thermos change its flavor?

So lately I bought a really decent coffee thermos that Fellow makes. It keeps drinks ridiculously the same temp for hours and hours. I drink longer shots of espresso, somewhere approaching an americano but not as watered down. So I put my drink in there and have been noticing that once it’s in there, it tastes ok but after an hour, it starts tasting burnt.

It actually has a ceramic interior so it’s not really stainless mixing its tastes in there.

I have a theory that it’s doing such a great job at keeping the temperature that it’s essentially cooking the espresso and giving it that burned taste by holding the temp at the same temp as what comes out. Like keeping your drink in those ember cups. What do you guys think?