Yeah, when a blocked date becomes randomly available it has a very 'gaslighting' effect on me ("did I really mess that up so badly?!?"), as well as being disconcerting for the would-be guest. It makes more work all around. Seems ridiculously glitchy--a reservation system that overrides blocked days? Not the greatest quality control.
I have had my setting on 'available 6 months out' and conscientiously check this regularly and manually override the 'available' dates that I know won't work for me. For instance, I always block Wednesdays. Then, zap!, they are back.
For lack of a better idea, I am going to change my availability setting to 'default setting is blocked' and see whether that eludes the problem they have with their software. Anybody have experience with that?