In the code of LogFmtRenderer there is:

value = f"{value}".replace('"', '\\"')

However if you are trying to render an escaped double quote like
"I want to render this \"string\" with logfmtrenderer"

it ends up rendering it as
\"I want to render this \\"string\\" with logfmtrenderer\"

\\" is wrong and won't be parsed correctly by a logfmtparser.