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