Often, the “IMEI repair” is actually just a corrupted filesystem. Do this first:
If the IMEI remains "null" after a full software flash, the issue is likely in the protected EFS/DRM partitions. This usually requires specialized paid software: Chimera Tool
Visit the XDA Developers forum for the Sony Xperia XZ3. The community has posted pre-patched QCN files (with dummy IMEIs) that you can manually hex-edit with your own numbers.
Before using advanced tools, try these basic steps to see if the IMEI reappears: Restart the device : Hold the button and the
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