So, I'm testing with the type-doulos.tex file containg this
\starttypescript [serif] [doulos]
\setups[font:fallback:roman]
\definefontsynonym [Serif] [Doulos SIL]
\stoptypescript
\starttypescript [serif] [doulos]
\stoptypescript
\starttypescript [doulos]
\definetypeface [doulos] [rm] [roman] [doulos] [default]
\stoptypescript
Changing 'roman' with 'serif' it worked for me (do not ask me why).
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