The editor wanted the reply to be a bulleted list, so he began his reply with *. (This is common in most votes and some other discussions, including Articles for deletion at the English Wikipedia.) The Reply tool added a superfluous : in front of the bullet list formatting.
What should happen
The Reply tool should have accepted the * and not added any other list formatting.
The rules for replies might need to depend upon whether the reply is the first one being indented.
T263902: Enable Reply Tool to use context-specific indentation syntax – let me pick the * or : style for a whole page