Nothing. It's a bug in macOS. Try Option+Power On and see if it's there (it should be).
You'll see when you boot from the external that the internal is missing from the startup disk preference pane, too... unless your partition is an issue.
Intel Mac UEFI (and maybe Apple silicon, too; I don't know) can't start up from a partition that starts past the 2TB point or so. As such, you should not partition a large drive with separate partitions. Use one partition and then separate APFS volumes within that partition. That way the BIOS limitations won't come into play.
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--Dave Nanian
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