govuk-font(14) outputs incorrect font-sizes when compiling with node-sass

Description of the issue

compiling styles with libsass (in this case via node-sass) causes govuk-font(14) to have incorrect font-size

this isn't related to decimal rounding, but because of some logic to suppress deprecation warnings not working exactly the same under libsass as it does when using dart-sass

Steps to reproduce the issue

gh repo clone alphagov/govuk-frontend
npx node-sass govuk-frontend/packages/govuk-frontend/src/govuk/all.scss node-sass.css

open node-sass.css look at what is created for .govuk-body-xs and anything similarly using the deprecated govuk-font(14) and you'll see output like this:

.govuk-body-xs {
color: #0b0c0c;
font-family: "GDS Transport", arial, sans-serif;
-webkit-font-smoothing: antialiased;
-moz-osx-font-smoothing: grayscale;
font-weight: 400;
font-size: 3.3125rem;
line-height: 1.03774;
margin-top: 0;
margin-bottom: 15px; }
@media print {
  .govuk-body-xs {
    color: #000000; } }
@media print {
  .govuk-body-xs {
    font-family: sans-serif; } }
@media (min-width: 40.0625em) {
  .govuk-body-xs {
    font-size: 5rem;
    line-height: 1; } }
@media print {
  .govuk-body-xs {
    font-size: 53pt;
    line-height: 1.1; } }
@media (min-width: 40.0625em) {
  .govuk-body-xs {
    margin-bottom: 20px; } }

where the font-sizes are wrong

seems like cause could be related to some logic in the sass around suppressing warnings for internal usages of govuk-font(14) where something is giving different result in node-sass and causing the logic to be incorrect and pick up the biggest size typography instead.

Actual vs expected behaviour

you can see the xs font size is showing as font-size: 3.3125rem; and on print for example, 53pt - which is bigger than headings, where it should be like this, which is from the dart-sass compiled output in govuk-frontend dist

.govuk-body-xs{
    color:#0b0c0c;
    font-family:GDS Transport,arial,sans-serif;
    -webkit-font-smoothing:antialiased;
    -moz-osx-font-smoothing:grayscale;
    font-weight:400;
    font-size:.75rem;
    line-height:1.25;
    margin-top:0;
    margin-bottom:15px
}
@media print{
    .govuk-body-xs{
        color:#000;
        font-family:sans-serif
    }
}
@media (min-width:40.0625em){
    .govuk-body-xs{
        font-size:.875rem;
        line-height:1.4285714286
    }
}
@media print{
    .govuk-body-xs{
        font-size:12pt;
        line-height:1.2
    }
}
@media (min-width:40.0625em){
    .govuk-body-xs{
        margin-bottom:20px
    }
}

Environment (where applicable)

only applies where anyone is compiling the govuk-frontend styles with libsass (for example, node-sass)

Operating system: not operating system specific
Browser: not browser specific
Browser version: not browser specific
GOV.UK Frontend Version: from when govuk-font 14 was deprecated I think, not sure the version

alphagov / govuk-frontend