Normally, when configuring HA FMC, your sensor should get a config with the 2nd FMC that will be used when primary fails.
If it's not working, then you might need to go back to your sensor, remove the registration and re-do it.
There was a link I already shared I believe in this forum. Here is the link with a quick and clear explanation:https://dependencyhell.net/2017/07/10/FMC-HA.html
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