Hey amanuay,
Thanks for the reply and additional information. We appreciate it!
Just to verify, are you following these steps to switch to another app from Notes?
Switch apps on iPhone X or later, and iPad
If you have an iPhone X or later, or an iPad:
1. Swipe up from the bottom to the middle of your screen and hold until you see the App Switcher.
2. Swipe left or right to find the app that you want to use.
3. Tap the app.
As long as you do not swipe Notes up and off the screen, the app should not fully close. That would be when reopening it would start fresh, instead of picking up where you left off.
Switch apps on your iPhone, iPad, or iPod touch
If you are not inadvertently closing the app, instead of switching, have you restarted your iPad since noticing this happening? If not, go ahead and do that, as a restart can resolve unexpected issues.
Restart your iPad
Test switching from and to the Notes app again, and let us know how it goes.
Take care!