Air back through the intake? Is it normal?

Assembling the engine and now have the lower intake, headers, valvetrain, and sparkplugs in.

I have the intake ports on the lower intake covered with masking tape to avoid debris from getting in.

When I rotate the engine, I noticed while watching the tape on the intake, that the ports suck the tape in some (normal), but that sometimes, I can also see where air is causing the tape to rise too.

Is that normal? Should air from the compression come back up through the intake port at all? If not, what is the most likely cause? Intake rocker a little too tight causing the valve to hang open maybe?

Thanks for the help

Also, I noticed that the quick pop up of the tape is just as the intake valve of each cylinder starts to open, then the tape is quickly sucked down. A natural result of cam overlap maybe?

Sorry if this is a very simple and basic question, I just never really noticed it before and as time gets closer to installing the engine, I get more and more paranoid.

They are used heads, but the machine shop checked them out and gave them a clean bill of health.