If you use the “backspace” key, it won’t work, but it works for me using the “Delete” key.
I use it for both modern airliners and smaller prop airliners, right up to the DC-6.
Atc always respects it.
I must admit not having tried mid numbers such as 500’s, as these are not normally used IRL.
I’ll give that a go today and see if that may be the issue.
I’ll let you know how I make out!