useSWRMutation cannot be used with fallback cache from SWRConfig

[yfhui]

Bug report

Description / Observed Behavior

I observe that if I provide a fallback cache to SWRConfig, using useSWRMutation will always return undefined and ignoring the cache config.

I saw that internally useSWRMutation reuse the middleware system and discovered that it does call useSWRNext under the hook. I think it could explain why this feature is lost.

Expected Behavior

I assume that useSWRMutation has a same behavior as useSWR so that if a fallback value is provided, then it should return the fallback value before mutate. It will be very useful in a scenario that a page is loaded by SSR but also there is a need for manual mutation to update the data at client side.

Repro Steps / Code Example

a simple example fork from swr demo
I expect the fallback data should be rendered first, but user can manually update the data as well with the triggering the button

Additional Context

This issue is obeserved at swr 2.0.0

I am not sure if this is an intended behavior or not but I hope that my judgement and understanding to swr is correct. Plz point out if I have missed anything. Thanks.