That was exaclty what I was looking for
This has been my second week working in Sketch, and I having the same trouble as you were. I coudn’t understand why styles already predefined were changed once I pointed a non-styled text. So I did the same as you by creating an extra page with my text styles manually. What it was such a great work for me to structure and define the styles for my interface, but I was doing unnecessary extra work.
I could not find any information about this, not in videos, not in blogs where they specify the facts.
I would like to thank you again for sharing your experience. It has been extremely helpful for me, even though it could look like a little stupid issue. Little changes are powerful.