Think the oil of bergamot essence just evaporated on storage.
" Earl Grey tea is a flavored tea, so my guess is that it is the flavoring agent that loses its strength over time, not the tea itself."
"Think the oil of bergamot essence just evaporated on storage."
Indeed!

i have frozen excess tea i bought in india on many occasions. the tea made was still very acceptable
Very acceptable With regard to tea? This will never do!
Feigned snobbery aside, it's never crossed my mind to freeze tea, gengis - but I might experiment...
The correct way to store any product that may lose its flavour over time, and this includes not only tea but ground coffee and spices, is to store it in non porous airtight jar, or a structure that can be made so. I once seen and oriental 'spice cuboard' that had a series of glass containers that had very tight lids, that you placed inside of other larger glass containers inside of compartments made for them..