iPhone 7 Plus Vibration Problem

Hi

After update to IOS 11, my phone vibration gote problem. I searching on the web and find some solutions to resolve this issue.

Finally, i rest my phone by selecting Erase all content and settings.

After phone registered and goes up everything was ok and vibration works correctly after 2 days vibration fails again and did not work.

I check other solutions but anything did not work, for the second time I erase all content and setting and rest the phone after the phone goes up everything works great and vibrations work fine after few hours vibration stop working and fail again

my phone model: iPhone 7 plus 128 GB

iOS Version: 11.2

What you think guys? is it hardware problem or software problem?

Why after rest to factory setting vibrator work fine?

Why after few days vibrator stop working?