Joining a mission doesn't always remove a star. The mission difficulties are bands of values, 35-55 is 2 stars for example. A 100 sided die is rolled with modifiers both from the environment (hinted at in the mission descriptors red and green text) and the squad. The Leader provides a modifier as well. It could be that the modifier is not enough to remove a star even though it is actually reducing the difficulty. Does that help?
I mean you very well could have a bug and in that case I do want to fix it!