With ConTeXt  ver: 2020.11.28 13:18 LMTX  fmt: 2020.11.29 the 
*\framedtext* output is consistently *0.8\textwidth*. MkiV gives the 
expected result.

    \starttext
    \framedtext{Fail}\par
    \framedtext[width=fit]{Fail}\par
    \framedtext[width=3cm]{Fail}\par
    \framedtext[width=0.8\textwidth]{Fine by accident}\par
    \framedtext[width=\textwidth]{Fail}\par
    \framed{Fine}\par
    \framed[width=fit]{Fine}\par
    \framed[width=3cm]{Fine}\par
    \framed[width=0.8\textwidth]{Fine}\par
    \framed[width=\textwidth]{Fine}\par
    \stoptext


-- 
Rik