I'm getting errors when setting appType: 'mpa' and requesting a URI from the dev server that doesn't exist (and would fall back to index.html in the SPA case). Instead of returning a simple 404, as expected, it's a hmr overlay that says:

Cannot read properties of undefined (reading 'charAt')

    at file:///usr/src/client/node_modules/vite/dist/node/chunks/dep-51c4f80a.js:59865:27
    at viteHtmlFallbackMiddleware (file:///usr/src/client/node_modules/vite/dist/node/chunks/dep-51c4f80a.j

I couldn't track back the code in that chunk to it's source (not sure which package this is from?), but it does this:

if(rewriteTarget.charAt(0) !== '/') {
	          logger(
	            'We recommend using an absolute path for the rewrite target.',
	            'Received a non-absolute rewrite target',
	            rewriteTarget,
	            'for URL',
	            req.url
	          );
	        }

Clearly, it expects the rewrite to return a string in any case. But the lines I commented on, will return undefined for mpa.

I can fix this by just returning an empty string in the mpa case. I'm not sure whether that's the right thing to do, though?

Commenting here, because this change introduced the bug.

Would you create a minimal reproduction and an issue in the repo so we don't lose track of this issue? You can link to this PR and comment, that is helpful to know already the connection.

Done in #10966.