subsetting multiple instances of a Variation Font fails in PDF output

[cairo 19c15ced]
[pango 239e030e4f38f0b7b899a8a405437601daf2860d]

My test font is NotoSerif[wght].ttf; the 'wght' range is [400;700], with named font instances at values 400 ('Regular', the default), 500 ('Medium'), 600 ('SemiBold'), and 700 ('Bold').

Now consider this script noto-test.sh.

pango-view \
  --markup \
  --text="\
<span font=\"Noto Serif @wght=$1\">foo</span>
<span font=\"Noto Serif @wght=$2\">foo</span>" \
  --output="noto-test-$1-$2.pdf" \
  --no-display

After calling

sh noto-test 400 700
sh noto-test 456 654

I get two PDF files, noto-test-400-700.pdf and noto-test-456-654.pdf. Both files don't contain correctly subsetted VF instances: The first file seems to display wght=400 two times, with slight metrics differences; the second one seems to display correct 'wght' values, however, the metrics are wrong, at least for the thinner instance.

noto-test-400-700:

noto-test-456-654: