Consider the idea that there is nothing wrong with the model.

The deck is connected to the pillar with a spring stiffness you defined in the bushing connection.

Modal analysis computes the natural frequencies and mode shapes of the mass-spring system.

The deformation is scaled to some arbitrary standard, which means the magnitude of the deformation is meaningless.

In addition, you can scale the displayed deformation to be much, much larger than the numerical deformation.

Connect the solution of the Modal analysis to the Setup cell of a Harmonic Response analysis and apply a load to find the true deformation for that harmonic load amplitude and direction.