I found this article by Philip Harben:
If you find in your thermos flask a strange liquid and you cannot decide whether it is coffee or cocoa, ten to one it is TEA which has been there for a long time with the milk in. Although tea keeps hot for a long time in a thermos flask, the flavour deteriorates. Tea is the most tricky and sensitive beverage and for perfection it must be freshly made. A curious and disagreeable flavour develops with keeping and – this is the important point, it is ten times worse if the milk is in the tea. Therefore if you must take your tea in a thermos, keep the milk out. take it in a separate vessel.
Lois Elsden 
A young girl at the office notices a thermos bottle on her friends desk and asks what it is. She is told it is a thermos bottle and it keeps hot things hot and cold things cold. The next day she comes to work with her own. Her friend asks her what she has in the thermos and she smiles and says two cups of coffee and an ice lolly.
LikeLiked by 1 person
Good grief! I can just imagine that happening!
LikeLike
Or take a separate tea bag and just keep hot water in the flask.
LikeLiked by 1 person
I carry teabags (in those little envelopes) in my bag… just in case!
LikeLiked by 1 person
Sensible. I always carry teabags abroad. Typically British 😀
LikeLiked by 1 person