Smooth changes to physical pressure should produce smooth changes to the pressure readings from a a tablet. When this is done well I use the term "pressure transition stability".
An unstable transition would mean that smooth changes in physical pressure would produce exaggerated or wild swings in the pressure readings. I call this "pressure transition instability"
No pen is perfect, but most are pretty good. However, it is not uncommon to discover a pen that has problems with these pressure transitions. Usually it happens in the lower range of pressure.
These unstable readings may not be visible if pressure used to alter the size of small brushes - for example 5px or 10px. So to see the behavior we need to scale up the effect by using larger brush sizes - for example 300px.
NOTE: Your hand itself will introduce imperfections in a stroke. So it takes some practice to learn how to distinguish the effects you are causing, versus what is happening with the tablet.
Compare the three cases below. The example on the far right has noticeable "pulsing" effect which is a common way this instability shows up.