According to strings in Android 15, it sounds like Android may force Bluetooth to turn back on a day after you turn it off.

Android's Bluetooth Absolute Volume is a useful feature for many users, but you may need to turn it off sometimes. Here's what it does and how to disable it.

Most Android users keep their phone's Bluetooth turned on 24/7, but you may want to reconsider doing that due to the possible security risks.