Some Movies are no longer in my Apple TV Library

Actually you are paying for codes. However, even if you legitimately redeemed it or bought an items on iTunes then that applies to the file, and you only have access to stream it in the cloud if it is available in the store. There is no guarantee that it will remain there and nowhere does it advertise indefinite cloud access.

If a provider makes a change and adds it back then it is no longer the same item you purchased. This is clearly outlined in the terms, and would also apply to any other digital platform. If you had downloaded and backed up those titles then you would still have them.

If a provider removes content from the iTunes store and/or adds it back with changes then it will no longer be available to stream. It is always recommended to download and backup purchases