As for the trust part, I would say that this is more being naive, which is always a surprise to me.
We dealt with the British during Brexit and the overall feeling is that we lost the deal (fishing, British disregard of the deal, Irelands as an open entry point, ...). This is probably just a gut feeling, but we tend as a nation to wake up the hard way - surprised that others are not friendly.
It is time to acknowledge that we are a small fish compared to the US, China and we need to find a way to deal with that, and not still live in the idea that the world is looking at is.
We comment on what we think is diplomatically callous without actually knowing if/when France was informed. I think that France's best option, domestically, is to act surprised in order to save face. What should they do otherwise, after losing such a huge deal in a lucrative region? Honest question. What would you recommend France do in response, even if they realized there were substantial issues months ago and a real risk of losing the contract?
There is a lot of speculation without anyone here likely knowing what the truth is.
The issue is about the manner in which the deal was announced. Newspapers describe the urgent calls from the AU officials to their French counterparts hours before the very public announcement by the US, UK and AU.
Diplomatically, that's a pretty callous thing to do to a partner. The ballooning costs may have been a factor in the decision, but it's more likely that the move is purely geopolitical. In these matters, there is also no guarantee that the deal with the US will end up cheaper at all.
The more important subtext here is the damage done to the US' image and trustworthiness as it's willing to do backroom deals to serve its interests by disregarding friends and allies. Not the end of the world, but not a very smart move either if you want to be trusted.