PCB trace height to width ratio
I think the problem is related to how boards are etched. The mask protects the copper from the top. The side walls of the track are not masked. The etching solution dissolves the sides of the trace at the same time it dissolves the copper between traces. This happens at the top first. So after etching, the side walls of the track will not be vertical. The track will be wider at the bottom than at the top. This variability means that controlled impedance can be difficult when the trace is relatively thick (heavy copper) compared to its width. This is why I would discuss with the PCB vendor. They will know if they can achieve 50 Ohms with this geometry.