Getting "Not found: /_app/immutable/assets/...." on Vercel deployment even though the file exists

[grantjayy]

In a production deployment on Vercel, I'm getting logs from my handleError function in hooks.server.ts saying that some routes in the .vercel/output/static/_app/immutable/assets/ are not found, even though they are in the folder. This also only happens on a couple of requests every few hours (on a small-medium site).

My setup is:

• @sveltejs/kit - 1.0.0-next.504
• @sveltejs/adapter-vercel - next

My handleError function is very basic and just logs the error.

Here is an example of a route that is not found:

Not found: /_app/immutable/assets/_layout-ff3a027e.css

Here are some screenshots showing the file exists:

The requests are also not coming from bots and are indeed real user agents.

I don't know if this is a bug or something else. Does anyone have an idea of what might be happening?