This bug was introduced in 1.2.5

When putting colors into the chat, for example §c, the text after it gets cut off, when you use the "last command"/arrow up key and you try to edit it.

Also, it appears that sometimes minecraft deletes the full line whilest typing. Yesterday i typed in
/slapper edit 17 addcommand rca

kit get rattata
and it cleared the line around "kit".

Already talked to Slicedlime/other developers about the issue