\sc gets different results when changing the default font. See attached PDF.

\starttext

\startbuffer
{\sc Use CaPs and SmAll caps (preserve casing)}
\stopbuffer

\getbuffer

\setupbodyfont[dejavu]     \getbuffer \crlf
\setupbodyfont[ibmplex,rm] \getbuffer

\stoptext

I believe that IBM Plex Sans has no smallcaps. Try

mtxrun --script fonts --list --info ibmplexsans

in your installation to verify.

--
Rik