Its a bit more complicated than that I'm afraid. I'm not a super expert on the mechanics, but in basic terms:
When a
Power enters turmoil it selects the systems it will potentially lose next week. It does this by effectively selecting the ones that will balance its CC in the easiest, most efficient, manner. That is to say, the most expensive base upkeep ones. If the
Power exits turmoil the following week, then it will keep those systems it pre-selected. If the
Power does not exit turmoil, it will lose those pre-selected systems regardless of whether they were fortified or not.
So, your action to fortify the system will have contributed to trying to avoid turmoil overall for the Power itself, but it does not save the system itself if overall turmoil remains (and I assume it does for Winters). Oh, and nothing over the requirement counts. So whether a system is fortified one over, or 1000 over - the result is the same. Anything over is 'waste'. Sometimes waste is good though....
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It can get quite complicated because the folks who try to direct the actions and strategy of their chosen Power may wish to purposely lose systems and enter turmoil. They may encourage over fortification of some systems, and no fortification of others, so people can get their merits, but they can still lose certain systems.
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Anyhow, if you want to know whats happening with Winters and learn more, I suggest you contact the folks who run the Winters Reddit.
https://www.reddit.com/r/EliteWinters