why can new bar data on Monday destroy my strategy?
I think he means because your strategy can not predict the future.
With regards to the downloaded data -- historical data doesn't change, right? It could be a horrific error on the part of your broker, but that is unlikely.
More likely you are mixing time frames, symbols or data horizons and that is why the graphs look different than before.
The downloaded data (in *.csv or *.json format) is a text file -- so, you can view it in any text editor. Better yet, there are many file comparison tools available for free or little cost on the Internet. Why not do a file comparison between the two data files to confirm whether or not they really are different? And if they are different, the file comparison will show you why and explain the mystery.