I could understand that but non of my blocked dates until March next year have a diagonal line through them, they are just greyed out as normal. It is only the blocked dates from March 2018 onwards that show the line.
Regarding dates that are only greyed, could it be that you have the "Dates unavailable by default" setting enabled in the "booking window" section of the calendar "Avaialbility" settings?
Dates that are unavailable due to that setting are just greyed. Dates that are manually blocked also have the diagonal line.