Brush auto-flatten does not work?

So ive been drawing with the brush and having auto flatten on. But it doesn’t work and its converting all individual brush strokes into symbols which is annoying and is creating bad lag. Is there something im doing wrong? I have to manually flatten them with the select tool or select all which is annoying.
Is it also possible to animate this way by using textured brush strokes frame by frame with no lag? Thanks!

Brushes and brush options have no way to create symbols, you are leaving some information out. In any case, it is usually not a good idea to flatten textured lines anyway as the texture flattening will produce undesirable results
(this works slightly differently in Harmony) due to the way that textures and transparent regions are combined in Animate/Animate Pro.

Be aware that textured stokes are much more demanding on your graphic card’s resources. You might improve your experience by using a single monitor instead of dual display and disabling the Aero Desktop theme (on Windows).

For your symbols problem… please explain exactly what you are doing. How are you creating your drawing element, how are you creating the cels (frames) of your drawing element to be able to draw in them?