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Hi,
I'm not aware about such an issue as a general problem. Please compare the Signal strength (read it) of your two phones set to 4G and connected to the same network at the same time in the same place in Settings --> About phone --> Status --> SIM Status
If the signal on your Z Pro is showing a lower value, then that's the reason of stepping down to 3G. Usually the system steps down to 3G from 4G even if the 4G network is available, if on 3G still a higher data speed can be reached. This behavior can be affected by all download, upload and access (ping) speeds. You can compare the 3G/4G data speeds in the same place with Speedtest.
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