While headphone is connected builtin speaker cannot be used?

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I am trying to use the live effect app, most of the things worked fine. However, when I plug my headphone in, no matter what combinations I use, the built-in speaker just wouldn't work, even with built-in speaker and built-in mic. Then, when I unplug my headphone, the built-in speaker starts to work again. So I was wondering if this is an issue with Oboe or is it an issue with the platform?

I am using One plus 5 - Android 10.