Created attachment 148273 [details]
here's a screenshot of the affected brushes, strokes made with "stabilizer" mode, sample count 70

SUMMARY
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The Pencil 2B and Ink Pen brushes in the MyPaint bundle produce rather polygonal strokes when using the "Stabilizer" mode even at the lowest "Sample Count" values.
"Basic" and "Weighted" smoothing modes are unaffected.
This seems to happen regardless of Input device, giving the same result with tablet or mouse.
This seems to happen with the default brush settings of said presets.
However, changing the affected brushes' Slow Tracking parameter to 0 seems to apparently fix the issue, although the Ink Pen brush then *suffers a side effect* from this workaround, producing a repetitive pattern all across a stroke

NOTE: If you are reporting a crash, please try to attach a backtrace with debug symbols.
See https://community.kde.org/Guidelines_and_HOWTOs/Debugging/How_to_create_useful_crash_reports
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STEPS TO REPRODUCE
1. Select Pencil 2b (mypaint) or Ink Pen (mypaint) brush
2. Select Stabilizer smoothing mode with a minimum to medium Sample Count value
3. Try making round shapes like spirals or circles, using swift motions

OBSERVED RESULT
Resulting strokes look oddly polygonal

EXPECTED RESULT
Smooth and round strokes, like every other brush does under the same circumstances

SOFTWARE/OS VERSIONS
Windows: 10 Pro Ver 21H1 (Build 19043.1645)
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Using Wacom CTL-472, driver ver. 6.3.46-1