It's completely normal and it's the bearings. There is always a bit of forward/backward play in bearings so when installing the flywheel simply hold the engine in the palm of your hand, slide on the collet, gently pull the the crank forward and slide the collet down against the front bearing inner race. You don't want to pull hard on the crank as you do not want to put a side load on the bearing (which it is not designed to handle) but you just want to make sure that the crank is snug against the bearing. Then install the flywheel and tighten the flywheel nut and you are done.