Tibet is occupied in a way that there is almost no historical precedent for. Tibet was a slave economy with a monarchy. China's occupation has increased the autonomy of the masses of Tibet over their previous formation. There have been very few, if any, occupational in history where the people being occupied run their own government, speak their native languages, maintain their cultural traditions, and collaborate with their occupier on programs to reduce poverty and increase quality of life.
But of course the reality is always multifaceted. In American occupied territories you don't see foreign governments training terrorists and air lifting them into the territory to create chaos and death. Whereas in Tibet that's exactly what the US has done.
There's a reason why the Dalai Lama's brother wrote in his memoirs that he deeply regrets collaborating with the Americans/CIA - because they made everything fundamentally worse and there only interest was attacking communism at all costs.
Further, Tibet never had any standing as a Westphalian nation-state. It was never recognized as a nation-state internationally. Now, I don't personally think the Westphalian system is a great system to make judgements by - Palestine has never been a Westphalian nation-state, for example - but we need to be clear in what our comparison points are. Tibet is occupied like Palestine and the Americas are occupied. Of those 3, Tibet is doing the best, has the greatest quality of life, has the greatest autonomy, has the safest cultural practices, and is on an upward trajectory.
Would I love to see decolonization everywhere? Yes. But I am fully able to hold that desire while realizing we have to decolonize the Commonwealth first if there are to be any unsustainable gains along that dimension. Any attempt to decolonize territory that is not part of the historical Commonwealth is going to be a movement that strengthens the EuroAmerica global colonial/neocolonial empire.
The majority of the Tibetan people are living good lives, are happy enough with the current situation, and are not agitating for separation, likely because they understand first hand what US and British meddling leads to - mass death. The Free Tibet and other similar orgs are Western orgs, not homegrown ones, and they primarily serve the purposes of US imperialism.