If the entire stroke is being undone then it looks like the stroke input isn't actually breaking up. It's possible that there's a pressure fluctuation causing the tools to suddenly act as if the pencil was being applied with no pressure.

It might be easiest to troubleshoot this via email. Could you get in touch with us via [email protected] and let us know if you use the Paint Roller whether you see breaks in the stroke, or whether the stroke varies in how much paint is being applied (higher pressure causes less paint to apply to the surface, lower pressure causes more so the grain shows through less). That should give me a good starting point for working out some steps to test this!