1. Open https://jsfiddle.net/Comandeer/ge4qu390/
2. Place the caret at the end of the second line.
3. Press Backspace.
The last character in the second line is deleted.
The selection jumps to the beginning of the paragraph.
If the reproduction steps are repeated with subsequent lines, the selection will jump to the last place of the split.
The splitting logic imitates what happens in CKEditor 4 during pressing Backspace. The error is not reproducible if the custom splitting logic is replaced with the native Text#splitText() method, but due to a bug in Chrome (https://bugs.chromium.org/p/chromium/issues/detail?id=1201161) connected with this method, I was searching for some workaround and discovered this issue.
It works fine in Firefox 88.0.
A similar issue is reproducible in Blink: https://bugs.chromium.org/p/chromium/issues/detail?id=1202439
I am able to reproduce this bug in Safari 15.5 on macOS 12.4. From the linked Chrome bug link, it was fixed in Chrome 91 and it was already working with Firefox 88.