How legacy? Personally I made this change/split maybe 12-15 months ago and still get the prompt. I'm also pretty sure I had a client with this same issue recently, although I can't confirm with certainty. I routinely migrate clients TO G Suite which is why I have some first-hand exp here.

Also, when signing up the Google system will not allow you to use the same email (if it's registered) but once you "take over" or verify domain ownership, you can then claim / use that address - which I believe creates this situation.