[NTG-context] odd behaviour with \definefontfamily

[jbf]

Can someone tell me why, when I define my typefaces as follows below, I get:

mkiv lua stats  > loaded fonts: 2 files: baskervaldadfstd.otf, 
texgyrepagella-math.otf

when in fact my font definitions are:

\definefontfamily [mainface] [rm] [baskervaldadfstd] 
[tf=file:BaskervaldADFStd.otf]
\definefontfamily [mainface] [ss] [librisadfstd] 
[tf=file:LibrisADFStd-Regular.otf]
\definefontfamily [mainface] [mm] [TeX Gyre Pagella Math]

\setupbodyfont[mainface]

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ConTeXt loads the [rm] and the [mm] but not the [ss]. Be assured that 
the ss typeface is available. So if, for example, I comment out the 
first line ([rm]) and change the Libris to rm instead, I am told:

mkiv lua stats  > loaded fonts: 2 files: librisadfstd-regular.otf, 
texgyrepagella-math.otf

In other words, only two, not three definitions are working at any one 
time. Or is it not accepting [ss] for some reason regardless? I have 
tried any number of existing sans serif fonts and none of them will 
load. I know the family names are correct, so that is not the problem. 
What could I be doing wrong?

Julian