Hyvä broke after an upgrade, can you fix it?

Yes, this is a frequent ask. After bumping hyva-themes/magento2-*, a compat module, or Magento itself, templates can 500 or render twice (Unable to resolve the source file for hyva_default). The cause is usually a theme override that drifted from the new parent template, two compat modules now registering the same block, or a renamed Hyvä layout handle. We diff your overrides against the new Hyvä version, re-base only what changed, and add a check so the next upgrade is cleaner. See also our Magento upgrade bug fixing service.