Version 4.3.1-alpha (git aa84881)

SUMMARY
When trying to use wet brushes on a Paint Layer with alpha-lock toggled on, no new strokes appear on top of pre-existing strokes, even if brush is set at 100% opacity. The Undo History shows that a "freehand brush stroke" occurred, but no change is seen on the paint layer. This problem only occurs when Alpha Locked is turned ON. When it is OFF, the Wet Brushes work normally.

Other brushes do NOT have this problem from what I can see, only wet brushes (Wet bristles, Wet circle, Wet knife, Wet paint, etc.)

STEPS TO REPRODUCE
1. Add a new Paint Layer with Alpha Lock property turned off (Alpha Locked: No).
2. Select one of the wet brushes like "Wet Paint" at 100% opacity, select a first color, then make strokes on the canvas.
3. On the same Paint Layer, turn the Alpha Lock property on (Alpha Locked: Yes)
4. Using the same wet brush and a different color, add new brush strokes on top of existing brush strokes made from Step 2 on the canvas.

OBSERVED RESULT
Brush strokes made when Alpha Lock is turned ON in the second color from Step 4 do NOT appear on top of strokes that were made with the first color from Step 2 using the same Wet Brush.
 
When the Alpha Lock is turned OFF and you try to draw on the same layer, strokes that you make after turning off the lock can be seen.

EXPECTED RESULT
New brush strokes using Wet Brushes in the selected second color should appear on top of the brush strokes made with the first color regardless of whether Alpha Lock is toggled on OR off. 

SOFTWARE/OS VERSIONS
Windows: Version 4.3.1-alpha (git aa84881)
Qt Version: 5.12.9
Surface Pro 3, Windows 10