Don't worry, it's fine!
I was also concerned that dates I block outside my availability window will become availability as the rolling window uncovers those dates - but that is not the case! It is OK!
See: https://www.airbnb.co.uk/help/article/3611
"Nights you manually mark as open or blocked won’t be changed
Once you’ve marked specific nights as open or blocked on your calendar, those settings will stay the same, regardless of your availability window."
I tested this by manually blocking some dates 7 months in advance that are obviously outside my normal 6 month window. The manually blocked dates didn't look any different from others that were outside the 6 month window. I then changed to a 12 month availability window and the dates I manually blocked were still shown as blocked, whilst those dates immediately before and after were now shown as open. Great! So I changed window back to 6 months, confident that my blocked dates will stay blocked.