A study of teabags sold in British supermarkets in 2011 found that the teabags containing the most L-theanine per cup (24 mg versus 8 mg per cup) were the lower-quality brands containing black tea, with a supermarket brand of black tea having the highest theanine content. The study demonstrates that brewing time is a major determinant of the amount of l-theanine extracted. Addition of sugar and small quantities of milk make no significant difference, while larger quantities of milk reduced the measured theanine content.
source: https://en.wikipedia.org/wiki/Theanine
Hm, i ask myself if the reduction in measured theanine is just by dilution or if the milk reacts with the theanine.