TikTok not capturing original video audio when sound is selected before recording — anyone else

Whenever I select a sound before recording a TikTok video, whether it's from the library or an original user-uploaded sound, the original audio in my video is completely gone. The slider to adjust the balance between original and added sound is greyed out and unusable.

However, if I record the video first and then add a sound in editing, everything works fine, the slider is there, and I can balance both audio tracks normally.

I need to be able to select the sound first so I can hear it while recording, but doing it that way kills the audio in my video entirely. Microphone permissions are enabled, and no microphone toggle is visible on the camera screen. Has anyone experienced this or knows how to fix it?

This has been happening since I first started using TikTok years ago, but I never cared because I never needed to select a sound before recording until now.