XPS 15 9570, Ctrl+Shift+Home/End, no longer working

Hi, I have a 15" XPS 9570 (c. March '19). Until recently, I was able to select all text in documents from the current cursor position to the beginning/end of the document by pressing Ctrl+Shift+Fn+Home/End. However, this stopped working about a month ago. Now, there is no way to do this.

I can still do the following:

Fn+Home/End to go the beginning/end of a line
Ctrl+Fn+Home/End to go to the beginning/end of the doc
Shift+Fn+Home/End to select from the current cursor position to the beginning/end of a line

However, pressing all four keys (the equivalent of Ctrl+Shift+Home/End on a full keyboard) no longer works. This is across all programs in which it used to work (e.g., including both MS Word + google docs). I've tried every possible combination of keypress order. I thought I might possibly need a BIOS update, but updating to 1.13.0 did not fix it. I'd love to get this working again. Any one else have the same problem or (better yet) a solution?