Yes, that's normal. The longer a chat gets, the more text the model has to keep in view, and the instructions you gave at the very start carry less weight against everything that came after. It isn't being stubborn, the early rules just get diluted.
What works well in practice:
- Repeat the rules briefly when you notice drift. One line is enough: "Reminder: short answers, no jargon, British spelling."
- Keep the rules in one place and paste them into a new message instead of scrolling back to find them.
- Start a fresh chat every so often and open with a short summary of where you got to, plus your rules.
- For work you repeat, save the instructions somewhere reusable (a Project or a saved prompt) so you never retype them.
A good rule of thumb: if you're past 20 or 30 messages and the quality is slipping, a fresh chat with a tight summary will usually give you better results than pushing on.