i checked - works fine for me on 2019. as musashidan says it's probably something to do with your specific file I had a quick check in the maxscript docs and there's an example rtt script here ...…
I was going to suggest switching to scanline as this is clearly some sort of horrible bug. I can't test it for myself atm but would be happy to once I get to a computer. Arnold will probably have a separate set of commands for RTT - it does in maya. If it's anything like in maya you'll probably have to knock up a little…