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It happens when I'm using either a Smear Stamp or Smear Source, and if I stop putting pressure on the tablet (no matter how quick or slow), it also happens with a mouse.
Today it was working fine, though I've had this happen since I got my license but wanted to make sure the topic wasn't repeated but I couldn't find any other.
Maybe the attachments will show it better (image + video).

I'm using a Mac OSX 11.2.1 (Big Sur), TVPaint 11.5.1 Professional (64 bit), my tablet is Wacom Intuos PT M.

You have the brush set to SMEAR SOURCE this means you can choose what the brush is gonna smear with,
instead of just smearing with he current frame.

If you open up the shape settings panel(windows>drawing>shapesettings) ; you can choose to set the source to either:
current
previous
next
display
paper
or
spare

So in this case i suspect what might be happening is that source is set to: previous, meaning that the smear will take its cue from the previous frame hence the unexpected result.

Changed the Shape Settings to try if it would change with frame order (I tried all of them), I altered the blend modes of the brush and restarted the program too but that didn't work either.