I'm using the waveshare 3.5 inch touchscreen, it's wiki can be found here 3.5wiki.
From the moment I use the apt repository it becomes blank at the next reboot. The raspberry stays responsive (tested with ssh). Using dpkg to install deb packages seems ok.
It does not matter if I install the screen drivers after or before using the apt repositories.
I will try to get some error outputs but I haven't got a sd reader at the moment so this might take some time.
I presume that it has to do with the files created upon usage of the apt repository but this is only my guess.
I found someone with the same issue but his problem lay with corrupted downloaded files which isn't the problem here.