I cannot always provide a minimal example (how much I would like to) as my TeX is produced in a pipeline from 5600 lines of lua and METAPOST code.
But luckily, in this case I can, the one using \phrasetext is indented, the other one is not.:
\setuplanguage[en][patterns={en}]\mainlanguage[en]
\definelabelclass[phrase]
\setupphrasetext[en][metamodelexplanation= {Only direct relations are shown. Abstract elements are expanded so only real metamodel elements are shown. Uses the Mastering ArchiMate 9-colour scheme to assist learning and quick reading.}]
\starttext
\setupindenting[none]
\setupindenting[no]
\definelayer[coremodel][width=\textwidth,height=\textheight]
\setlayer[coremodel][preset=lefttop,voffset=0.6cm]{\framed[frame=off,align=flushleft,width=\textwidth]{\phrasetext{metamodelexplanation}}}
\setlayer[coremodel][preset=lefttop,voffset=6cm]{\framed[frame=off,align=flushleft,width=\textwidth]{Only direct relations are shown. Abstract elements are expanded so only real metamodel elements are shown. Uses the Mastering ArchiMate 9-colour scheme to assist learning and quick reading}}
\placelayer[coremodel]
\stoptext