I can reproduce with the palatino on my system which comes in ttc-format.18. November 2016 um 23:08
I don't know,
\definefallbackfamily[mainface][serif][texgyrepagella]
\definefontfamily[mainface][sans][texgyrepagella]
\setupbodyfont[mainface]
\starttext
Why is the text in bold italic?
\stoptext
works ok here so you need to check what palatino gets selected (maybe you need to be more specific)