[XeTeX] XeLaTeX generating Y with grave Ỳ (U+1EF2)

[Ken Moffat]

I'm revising some PDFs which each look at a single font, using
XeLaTeX, and I've hit a gotcha which I had not expected.  I show all
the letters of selected Latin alphabets, one of which is Welsh
because of its wide range of accents, and I'm revising my treatment
of Old Standard (OldStandard-Regular.otf).

The Welsh alphabet appears fine, but amongst the large amount of
output are reports for missing codepoints.  That in itself is normal
because I finish the document with showing currency symbols and
other symbols, whether or not they are present.

But just before the missing currency symbols I noticed:

Missing character: There is no Ỳ (U+1EF2) in font Old Standard Regular/OT:scrip
t=latn;language=dflt;mapping=tex-text;!
Missing character: There is no ỳ (U+1EF3) in font Old Standard Regular/OT:scrip
t=latn;language=dflt;mapping=tex-text;!

Which is true, my listing of the codepoints omits those. And yet the
alphabet is displayed, including Ỳ and ỳ.

I wonder how many similar issues I had overlooked.

What I don't understand is why the letters were rendered - I am used
to spaces, or indications that a codepoint is missing. Is there
logic somewhere which says that (maybe only a limited set of)
accents can be produced using combining accents even where the
precomposed codepoint is used ?

This is TL2024 and I'm also using polyglossia although I doubt that
could be involved.

TIA

ĸen
-- 
Voluntary positions are like garden bindweed: untended, they will
fill any void you might have.           -- James Max