The way I interpreted what happened in episode 16, is that the ML was not in his right mind to communicate clearly with the FL. Even though so far their communication has been really open so far, it shows that he still had reservations in telling her everything, and this unfortunately created a misunderstanding that was later called out by the FL. I have seen comments lambasting her for the way she treated ML, but considering what he did to her I think it’s natural for her to be afraid of him even though she understands why he did it. Forgiveness took time, which I appreciate and even then she later realized how hurtful her words were. Her responses are flawed, and most importantly human and she acknowledges it eventually and apologises and that’s what really elevates this story for me. Including the ML, even though he is an immortal who has been around for some time when it comes to inter personal relationships he still has a learning curve and will make mistakes. Yes, they have established a relationship where they are open with one another but that does not necessarily mean either one will get it righ all the time, and it was later addressed. So, I do not think it was “out of character” or a plot hole but a demonstration of how flawed they are. However again that is my interpretation of it, but I think for anyone considering on dropping it because of that just know it does get addressed later on and is not a moment that happens and is forgotten about