characters lost after multi-cursor undo

## Description of the problem or steps to reproduce

[![asciicast](https://asciinema.org/a/rWu9xwZjRPycFNkEgiORjjjdj.png)](https://asciinema.org/a/rWu9xwZjRPycFNkEgiORjjjdj)

## Specifications

Commit hash: v1.3.1
OS: NixOS
Terminal: lilyterm
