According to various dictionaries, sanguine means “(eagerly) optimistic, confident, cheerful, hopeful”. Yet I always get the impression that modern writers intend a somewhat weaker meaning: more like unworried or unperturbed, perhaps expressing the tone of “it’s going to be rough, but we can handle it” rather than “it’s going to be good”. What exact shade of meaning on this scale does the word express?
I will confess to having used and understood “sanguine” as described in your weaker meaning. I do agree that the dictionary meaning is much stronger: actually being confident and hopeful, whereas I had intended to express that I was unperturbed.
This link gives an example of someone explicitly using the lesser meaning.
My conclusion: I will not in future use “sanguine” without further clarifying my meaning. It is pointless to swim against the tide of language evolution, but I don’t want to be misunderstood. If I intend the lesser meaning then a reader may look the word up in a dictionary, if I intend the dictionary meaning too many folks now have adopted the lesser meaning. Can’t win, so use something else.