I haven't done extensive testing, so the problem may be bigger than just internal records. But this fails with an exception:
open System.Text.Json
open System.Text.Json.Serialization
type internal T = { foo: string }
let json = """{ "foo": "" }"""
let private options = JsonSerializerOptions()
options.Converters.Add(JsonFSharpConverter())
JsonSerializer.Deserialize<T>(json, options) |> ignore
If I remove internal from the type definition, it works.
This worked fine in 0.19.13.