Transformer’s Flux and Back-EMF
Before you connect a secondary load, magnetization current is flowing in the primary and this sets up the flux in the core. That flux remains constant as an RMS value with any load changes.

Any secondary load takes current and this creates a flux that is cancelled by the extra flux created by the primary winding due to that secondary load current. In other words load currents produce opposing fluxes that cancel and, all that remains, is the original magnetization flux.

This means that back emfs or induced voltages remain as they always were when there was no secondary load. Small print: this explanation assumes leakage inductance and winding losses are negligible.