3449995 - Build fails due to exception from uncaught JavaScript throw: Error: ParseError: Unrecognised input - SAP Commerce

While trying to create a custom theme based on an existing one, when importing a custom font, the build fails with an error similar to:

[apply] js: "path/to/core-customize/hybris/bin/custom/customstorefront/web/webroot/WEB-INF/lib/lessc-rhino-1.7.3.js", line 61: exception from uncaught JavaScript throw: Error: ParseError: Unrecognised input in path/to/core-customize/hybris/bin/custom/customstorefront/
web/webroot/WEB-INF/_ui-src/responsive/themes/vestis/less/component/_fonts.less on line 6, column 14:
    [apply] 5 @font-face{
    [apply] 6   font-family='Montserrat';
    [apply] 7   src: url('../../fonts/Montserrat-VariableFont_wght.ttf') format('truetype');

SAP Commerce

commerce, custom, font, exception, uncaught, JavaScript, throw, ParseError, Unrecognised, input , KBA , CEC-SCC-CLA-BUI , Build Process , CEC-SCC-COM-BBA-ACC , Accelerator , CEC-SCC-COM-BCA-ACC , B2C Accelerator , Problem