To reproduce:

1. Start fish as fish --no-config -C 'fish --no-config --init-command='fish_vi_key_bindings; bind ctrl-c cancel-commandline; bind --mode insert ctrl-c cancel-commandline; bind --mode visual ctrl-c cancel-commandline'
2. Enter some characters, e.g. asdf
3. Press escape to get into normal mode.
4. Press ctrl-c to cancel the command line. Observe that the last letter gets overwritten, turning asdf into asd^C.

The same happens when pressing ctrl-c in visual mode. In insert mode, the desired behavior (turning asdf into asdf^C happens).

Tested on b0be53e.