Why is my iPhone XR left speaker not working?

I am sure you mean left speaker stereo speaker that is on top of the phone (ear piece speaker). I have the same issue. It shuts off randomly. I have to reboot the phone to make it work again. It's fairly wide spread issue and after reboot it will come back. Apple needs to fix this issue on next update.

At the bottom of iPhone, there's only one speaker (on the right side)

The other speaker is at the top (left side of the front facing camera)

The left opening is a microphone, not a speaker.