Can encode a map[int][whatever type], but cannot decode into the any type. Get invalid code=1 decoding string/bytes length, only in v5, works in v4

[element-of-surprise]

Was doing some decoding on a weird message that requires using any. Ran into an issue where I was able to encode the message but not decode it. Tracking it down, I was able to isolate it into code that I have running on the go playground:

https://go.dev/play/p/GbriQkE8RqN

I've then been able to isolate it further:

https://go.dev/play/p/FAhH3EckyEi

The essence is, if you encode:

map[int]string{"hello"}

You cannot decode out to the any type:

var out any

if err := msgpack.Unmarshal(b, &out); err != nil {
	panic(err)
}

This results in:

panic: msgpack: invalid code=1 decoding string/bytes length

This will work if you decode to a non-any type (such as Msg). So it is some type of interaction there on the encoder or decoder. It will also work if you change the key type to a non-integer type, like map[string]string.

If you decode into a concrete type, then this works as expected.

Expected Behavior

If you switch to v4 instead, you can see the correct behavior.

Current Behavior

It returns an error: invalid code=1 decoding string/bytes length

Possible Solution

I'm sure there is something in the decoder that isn't reflecting right or something in the encoder not encoding correctly.

Steps to Reproduce

https://go.dev/play/p/FAhH3EckyEi

Context (Environment)

Trying to test an encoding on some msgpack being sent on a net.Conn .