It is possible (and not that improbable: I discovered this while sketching) to start a stroke immediately after pressing / to switch presets, and have the stroke use shape, size, etc. from the new preset, but the blend mode from the previous preset.

Doesn't matter what the brush blending-modes are, though of course it is only observable if they are different.

I suspect using / to switch to the previous preset is an important part of this bug only in that it permits you to begin a stroke very soon after switching presets.

It might be difficult to reproduce on a high end machine. If so, maybe try with some CPU-intensive processes in the background.

Reproducible: Always

Steps to Reproduce:
1. Select two different presets which use different blending modes, one after the other. (e.g. Basic_erase and Basic_ink_ballpen)
2. Press /
3. Begin a stroke without delay!
Actual Results:  
The stroke has the properties from the new preset, but uses the blending mode of the previous preset. (e.g. shape and size of Basic_erase, but paints colour using normal mode instead of erasing with erase mode.)

Expected Results:  
The stroke is done either with one preset or the other. The thread getting the preset information for the stroke should not be able to access the info while it is still being modified by another thread / Blend mode should be switched simultaneously with the other properties (well it should appear that way, i.e. no access to the current preset information between setting the brush and setting the mode).