You sort of answered your own question. The stem binds if the locknut is tight, so install it first, then tighten the locknut.

The issue is concentricity. When tightened, the locknut isn't perfectly concentric with the ID of the fork. This is fairly common, and nothing to worry about. But if the fork ID and locknut ID are both close fits for the stem (as they should be), they'll slide on individually, but not as a pair when one is slightly offset (not concentric).

Minor issues of alignment are common in all sorts of assemblies, and your bike is no different. That's why many assemblies call for inserting and attaching ALL hardware before tightening any. Failure to follow that sequence is what has people throwing fits when the last nut and bolt won't fit through the (now) misaligned holes.