With the latest LMTX standalone (ConTeXt  ver: 2021.06.18) the Syriac text
is not printed.

\setupdirections[bidi=global, method=unicode]

\definefontfeature
   [syriac]
   [syriac]
   [language=dflt, script=syrc]

\definefallbackfamily[mainface][rm][Estrangelo Edessa]
[range={syriac},
features=syriac]

\definefontfamily[mainface][STIX Two Text]
\setupbodyfont[mainface, 18pt]

\starttext
Syriac

{
ܐܝܟ ܟܝܢܐ ܕܐܐܪܣ ܡܲܥܕܢܘܬܗܘܢ ܘܡܲܥܒܿܕܝܢ ܩܪ̈ܒܐ ܘܚܣܡܐ ܠܗܠܝܢ
}
\stoptext

The same is true if I use
\def\Syr{\definedfont[name:estrangeloedessa*syriac]}
and insert \Syr before the Syriac text.

I have the same issue with my Arabic font fallback but not with my Greek
font fallback.

Alan