I think it might help if we had a bit more ATC and Engineer input here.
I have never heard before of any transponder that automatically went into stby when the numbers are changed, equally I have been told that by going to stby one can disappear from the screen and computers being computers, one may not always reappear as required. As it was during a visit to LATCC that we, as a group of pilots, were specifically asked NOT to select stby I have never done it since, (over twenty years now).
Any ATCOs and Radio Engineers care to comment please?