This has been reported downstream in alacritty/alacritty#3323 and is a regression caused by 9daa073.

It seems like the fix for certain keyboard shortcuts not triggering ReceivedCharacter events was a bit over-eager and resulted in existing events that were working to suddenly send incorrect characters.

So the combination of shift+ctrl+6 now sends just 6 as character, instead of the desired \u{1e}.