It is intentional that the game uses your device's microphone instead of your headphone's. You can check out this reply by Tencent for more information, @꧁ᴋɪʟʟᴇʀ々ʏᴛ࿐.
You can check if covering your device's microphone with your hand causes this issue. If that is not the cause, feel free to report this issue through https://pubgmobile.live/support. Check out this announcement for more information regarding the bug reporting process.