Hello @BGMS, let me give you a warm welcome to the Fitbit Community. Note that the phone model you have mentioned is not listed as compatible device. With this I'm not saying it wont work, the difference is we cannot guarantee it will work correctly or what would be the behavior of your phone when is connected to your tracker like is your case.
Is not require to keep the Bluetooth On for your tracker work, however it will be require to manually sync your tracker or other feature that require a constant connection with your tracker will not work.
Although by the description you provided in your post, It might be a feature or an option of your phone that might be possible to disable directly from the settings of the mobile device.
Let me know if you have additional questions.
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