You get gold after you complete the whole stay that includes the 40th night and it posts to your account. therefore, if your 40th night is part of the same booking as your other 9/10 nights you won’t get gold until you complete the whole stay. I’d also suspect if it’s the day after you start your next stay you might not show as gold at checkin either as can take a few days to post / change.

Doesn’t really matter for you but for anyone else reading this if you were doing that number of nights an AMEX platinum card or if you’re in the USA one of the Hilton cards would’ve been a worthwhile investment.