JsonUnmappedMemberHandling.Disallow has no effect when using JsonFSharpConverter

[cmeeren]

Repro:

let options = JsonSerializerOptions(UnmappedMemberHandling = JsonUnmappedMemberHandling.Disallow)
options.Converters.Add(JsonFSharpConverter())

type A = { X: int }

JsonSerializer.Deserialize<A>("""{"X":1,"Y":2}""", options)

Expected: The last line should throw System.Text.Json.JsonException: The JSON property 'Y' could not be mapped to any .NET member contained in type 'A'.

Actual: Deserializes without throwing.

Removing options.Converters.Add(JsonFSharpConverter()) produces the expected result (throws).