No solution, just another oddity...

\definefontfamily [mainface] [rm] [TeX Gyre Pagella]
\definefallbackfamily [mainface] [mm] [TeX Gyre Pagella Math] [range=uppercasedoublestruck]
\definefontfamily [mainface] [mm] [Neo Euler]
\setupbodyfont[mainface,12pt]
\starttext
\startTEXpage[offset=3pt]
Test: $𝔺$ or $\utfchar{"1D53A}$ or $\blackboard C$ or $\mathbb C$

\switchtobodyfont[pagella]
Test: $𝔺$ or $\utfchar{"1D53A}$ or $\blackboard C$ or $\mathbb C$
\stopTEXpage
\stoptext

On Fri, 2017-11-24 at 13:07 +0100, Mikael P. Sundqvist wrote:
> Hi!
> 
> I use the Neo Euler which has no doublestruck characters. Maybe I use old code, but I really think
> that the following has worked before:
> 
> %%%
> \definefontfamily [mainface] [rm] [TeX Gyre Pagella]
> \definefallbackfamily [mainface] [mm] [TeX Gyre Pagella Math][range=uppercasedoublestruck]
> \definefontfamily [mainface] [mm] [Neo Euler]
> \setupbodyfont[mainface,12pt]
> \starttext
> \startTEXpage[offset=3pt]
> Test: $\blackboard C$ or $\mathbb C$
> 
> \switchtobodyfont[pagella]
> Test: $\blackboard C$ or $\mathbb C$
> \stopTEXpage
> \stoptex
> %%%
> 
> I would expect doublestruck letters in the output on the first line. The resulting file, compiled
> with a fresh Standalone, is attached.
> 
> Best regards, Mikael
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