Maybe you weren't all that good with qwerty before? I always felt it's like learning an extra foreign language: learning French doesn't make your German worse.
Also happened with language learning. I speak English and Spanish natively. Learned Italian (very similar to Spanish) in high school. After college learned Chinese in China. One day I ran into an Italian person in China and tried to talk to them in Italian. I kept jumbling Italian and Chinese, even though on paper Italian is so much easier for me than Chinese. It was like my brain only had space for all native languages, and just one foreign language. I’m sure with practice I could speak both Italian and Chinese, but that’s the whole point of this comment. It takes practice to maintain them all.
If your speed did not fall it means you did not learn well.
I actually learned to type at school (yeah, funny business school my parents sent me to). We had an actual specialist teacher to teach us to type on electric typewriters using pretty nice methodology.
To learn to type well is to train "muscle memory". This means you think the letter and your body does it for you, automatically, without any part of your conscious brain involved.
I don't know where keys are on my keyboard. It is unmarked, so I can't look it up. If somebody asks me, I need to lay my hands on it and think the letter and see which finger "wants to" press the button. In that regard I am a passenger and once I think a word or sentence I don't have any further control -- it is some uncontrollable part of my brain that takes over and does it for me.
Even when typing at speed I am still free to think about anything else. Just as when you walk you don't need to think about walking itself and can concentrate on whatever else you want.
The only exception to this is typing on my mobile device, which is configured to qwerty.
So maybe GP was that good