Hi! Your problem is called Linea Alba buccalis. It is the white discoloration of the inner buccal mucosa in the line of occlusion of the upper and lower posterior teeth. Since the problem started after the dental treatment and the placement of artificial teeth, the reason may be malocclusion
or the insufficient occlusion between the newly inserted teeth and the natura teeth. The cheek mucosa in this case gets irritated and traumatised leadind to increased keratinization and the appearance of white lining in the occlusion area.
Selecting grinding of the artificial teeth or the removal of any occlusal irregularity can resolve this problem. This is not a very serious problem but continous irritation can lead to more complications like Leukoplakia