“Disabling” Bluetooth (and WiFi) has been the norm for some time since iOS 11 in 2017 When Control Center got a major redesign and the functionality of the WiFi and BT button changed from “On/Off” to “Disconnect all connections, but leave radios on” to communicate with your watch, determine location more accurately, etc.

Shortly after release, it was discovered that disabling Wi-Fi and Bluetooth connections through Control Center did not disable the respective chipset in the device in order to remain functional for background communication, a design decision that drew criticism from “misled” users and reduced security due to potential vulnerabilities. In open, inactive connections

For convenience’s sake, I’ll assume you also do it this way, and in theory disabling BT in settings also means BT no It turns on again automatically. but:

If Bluetooth is turned off, you won’t be able to connect your iOS or iPadOS device to your Bluetooth accessories until:

I turned on Bluetooth in Control Center.
You can connect to a Bluetooth accessory in Settings > Bluetooth, by tapping the AirPlay icon or selecting the Bluetooth accessory.
It’s five in the morning local time.
You have restarted your device.

Every iOS update involves restarting the device, as well as Bluetooth (and Wi-Fi). Who designed it? Run again.