Link to the code that reproduces this issue

https://github.com/ninjapayofficial/Ninjapay-Plugin-Backend

To Reproduce

Here’s what’s going on under the hood in your installed Next.js (15.5.9).

1. Google Fonts catalog vs. fallback metrics (two different things)

next/font/google does not only “check Google.” It uses two separate pieces of data:

So the font is available on Google Fonts as Montserrat Underline; that’s why Montserrat_Underline from next/font/google can still download and work. The message is not “font missing from Google.”

2. Why the error is thrown

Fallback metrics are resolved in next/dist/server/font-utils.js by turning the family name into a camelCase key and looking it up in capsize-font-metrics.json:

function calculateSizeAdjustValues(fontName) {
    const fontKey = formatName(fontName);
    const fontMetrics = capsizeFontsMetrics[fontKey];
    let { category, ascent, descent, lineGap, unitsPerEm, xWidthAvg } = fontMetrics;

For "Montserrat Underline", formatName produces montserratUnderline.

In capsize-font-metrics.json, there is no montserratUnderline key. There is a montserratSubrayada entry whose familyName is "Montserrat Subrayada" — the older Google Fonts name for the same design:

  "montserratSubrayada": {
    "familyName": "Montserrat Subrayada",
    "fullName": "MontserratSubrayada-Regular",
    "postscriptName": "MontserratSubrayada-Regular",

So: Google renamed / rebranded the family to “Montserrat Underline”, but Next’s bundled Capsize metrics file still keys it under the old name → lookup for montserratUnderline is undefined → destructuring throws → get-fallback-font-override-metrics.js catches and logs:

function getFallbackFontOverrideMetrics(fontFamily) {
    try {
        const { ascent, descent, lineGap, fallbackFont, sizeAdjust } = (0, font_utils_1.calculateSizeAdjustValues)(fontFamily);
        // ...
    }
    catch {
        Log.error(`Failed to find font override values for font \`${fontFamily}\``);
    }
}

That matches reports like vercel/next.js#47115 / vercel/next.js#82079: the font exists on Google Fonts, but precalculated override metrics aren’t there for that identifier.

3. What you can do

• adjustFontFallback: false — skips this lookup; no more error; possible small layout shift when the webfont swaps in (same workaround as in your app/ninx/page.tsx).
• Wait for / contribute to Next — ideally they’d add an alias montserratUnderline → same metrics as montserratSubrayada, or regenerate metrics for the new family name.

Bottom line: The package does know about Montserrat Underline for fetching the font; the error is only because the fallback adjustment table still uses the old Montserrat Subrayada key, not Montserrat Underline.

Current vs. Expected behavior

Bottom line: The package does know about Montserrat Underline for fetching the font; the error is only because the fallback adjustment table still uses the old Montserrat Subrayada key, not Montserrat Underline.

Provide environment information

Node: 22+

Which area(s) are affected? (Select all that apply)

Not sure, Performance

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local), Vercel (Deployed)

Additional context

No response