At times, end users are used to working with Advanced Find a lot without having much control over what they can customize. Because for obvious reasons, they are users. And not someone who customize the system.
In this case, they might also not know what they can’t search for in the Advanced Find and are led into believing that all the fields are being shown.
And they could even wonder why it doesn’t appear in Advanced Find when they can see it on the Form/View.
However, there’s a configuration which can either hide/show the fields in the Advanced Find.
Making Fields Searchable
So this is the answer. If fields are not Searchable, they don’t appear in Advanced Find
Advanced Find field list with the missing field
And, when the Searchable is set to Yes, they appear in the Advanced Find.
Advanced Find showing the Searchable enabled field
So that’s why. Hope this helps!
D365 Demystified 
I want to make a field searchable but it’s “locked”, by default it is set to “No”.
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If it’s a system field then you can’t do anything about it. Microsoft locks down fields which are vulnerable to security. Can you let me know which field is the one you are trying to make ‘searchable’?
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