At first, an ergonomic chair might seem stiffer or more built than what you know and that’s fine. Proper support is what your body is adjusting to now.
Time for adaptation to a healthier position your muscles may need. This is true if your previous chair allowed you to be able to slouch or lean in such odd ways. At first, the upright posture or back support might be feeling a little unusual though not actually uncomfortable. A proper pair that supports your feet is like switching from soft, worn-out shoes.
Most people adjust after a few days and start to notice that they’re less sore. They also become much more alert, so they stop fidgeting just as much. That sign is the chair doing its job.
Yes of course, an ergonomic chair will then feel different, so it is in fact more supportive and more structured, and it is ultimately more comfortable once you settle into the chair.