how do I stop apple music from changing bit rates

I recently reloaded my whole CD library (luckily only about 500) to ensure that I have all high bit rate copies of songs. That was a month ago and today I am seeing songs in that library that are 256. What gives Apple?

My questions are what exactly causes AM to change the bit rate for some songs? Is it songs that have been downloaded to a device, even though all of our devices say not to change the bit rate? Is it random?

And, how can I stop this from happening? If it cannot be stopped, I'd be open to recommendations for other music services that provide higher bit rate files.