When using for example the button peripheral and central examples in the arduinoBLE library
If for example the central is a MKR1010 powered by usb and the peripheral is a nanoBLE 33 then if I remove the power from the central the peripheral still reports it as connected.

If instead of a Nano BLE 33 I use a Nano IOT33 then when the central is de powered the peripheral reports it as being disconnected.

there are several issues regarding disconnection & this library when using the Nana BLe 33.

Arduino support said the following

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Hello Russell,

Thank you for reporting us this issue,

We request you to open an issue on Github here and our developers will provide you further guidance.

Have a nice day!

Best Regards,
Sravya Amirisetti