u2270 antenna design hints
Use the general rules for RF PCB design: short connections, the connections that are still long should enclose very small areas, use decoupling capacitors placed very close to the U2270.
Since 125kHz is quite low frequency, use for decoupling something like 100n parallel with 1uF and 10uF,very low ESR and place them close to the U2270.
On the PCB layer closer to the antenna use a ground plane, or copper pour, hash.
Maybe the problems are due to the uC part of the circuit exposed to powerful field. Check the decoupling of the uC, and the data lines between U2270 and uC exposed to the field.