Help needed for understanding placement of flexible PCB GSM antenna
You go on about interference but that is not an issue.

To have what is called interference in the RF world, you need something else transmitting a signal, the signals then influence each other and disturb reception of these signals.

What you are doing will affect the radiation pattern and impedance of the antenna. For both non-flexible and flexible antennas the optimum radiation pattern (the one you see in the datasheet) is achieved when the antenna is used in free space. This means no conductors nearby. This "nearby" is often assumed a couple of wavelengths distance.

You do have conductors nearby, the traces on the PCB, the shielding can of the module etc. These will affect both radiation pattern and impedance. This will results in degraded sensitivity. Your module will still work but it needs to be closed to the base station.

Many mobile phones use flexible antennas near metal objects (shielding in the phone etc.) so it is common practice. But during design this influence is taken into account using EM modelling software and measurements.