Hello Tahaaaa,
Thank you for posting in the Apple Support Communities. We understand that music doesn't sound right when streaming multiple music apps to a Bluetooth speaker, and we'd like to help.
First, consider whether this happens with streamed or downloaded content. If the content is streamed, try downloading the songs to test the issue further.
Next, we suggest checking your sound settings and removing accessories from your iPhone. These steps can help ensure that nothing is accidentally adjusting the sound. This support article explains these steps in more detail:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
Also, test the issue without streaming to the speaker. Does the same thing happen only when playing music from the iPhone speaker? If applicable, try pairing another device (i.e. another iPhone, Mac, iPad, etc.), to the Bluetooth speaker, then playing music from both apps to further isolate the issue.
We hope this information is useful, but if the issue persists after taking these steps, feel free to reply back to let us know the results after trying. The more information you provide, the better chance the community will be able to offer some additional guidance.
Kind regards.