Oof, thanks. Do you think you can extend the unit tests to cover this case?

I could add a test that uses a binary.Unmarshaler implementation of a slice type. Kinda like in my reproducer but without the global counter variable. Might be a bit hacky, but should be fine for a test, I guess?

Do you think this should be a new separate test? Or do you have an existing test function in mind that should be extended with a test case for this issue?

I have added a new test function that tests key and value unmarshaling with a custom binary.Unmarshaler. Without the fix it fails the unmarshaled values assertion:

--- FAIL: TestMapBatchLookupCustomUnmarshaler (0.00s)
    map_test.go:1040:
        error:
          values are not deep equal
        diff (-want +got):
            []uint8{
            	0x03,
            	0x04,
          - 	0x05,
          + 	0x03,
            }
        got:
          []uint8("\x03\x04\x03")
        want:
          []uint8("\x03\x04\x05")