On the one hand, you’re right—it’s frustrating any time there’s a foreseeable and necessary turn/reversal of any kind in a narrative that gets abruptly thwarted; it’s the equivalent of rolling “go back three spaces” in a board game.
But I don’t always find it so objectionable as an element of characterization—at some level Landlady-san wants to confess, but can’t bring herself to do it just yet. (If she really wanted to, she could have done so once the interruption was over. So she obviously doesn’t.)
If the trope is a deal killer for you, that’s that. But I tend to think of partially-blurted-out confessions like this as basically being contained in a virtual thought balloon—readers will know how close the character came to actually confessing, but the other person doesn’t.