How to deal with a "preferred user setting" and automatic fallback

Context

I'm adding a new option in a configuration screen. It's basically a simple radio-button / combobox / checkbox that either uses the default Setting A or the new Setting B.

Setting A is the current default setting and guaranteed to always work.

Setting B is a new setting that adds some additional functionality, but isn't always guaranteed to work.

GUI example:

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A requirement is that if setting B doesn't work, that the system automatically uses setting A. I cannot detect beforehand if setting B will work unless the setting is saved and the system is rebooted (which takes about 10-20 seconds). This prohibits me of simply hiding the option if it's not relevant, or having some sort of "test setting" functionality.

Question: How do I make it clear that if setting B is selected, it might not work and/or will revert to setting A, BEFORE the user actually saves/reboots?

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