When using SharePoint as your data source and the built-in Form control, any choice or lookup columns will be added as a ComboBox (dropdown). If you have the Allow searching property set to true for this column, then when you search, it will only return values that start with the characters you entered. It will not search the entire text.
To demonstrate, I created a list named "Demo List" and it contains a choice column called "President". Each U.S. President is listed as a choice-value in the President column.
Searching will only return values that Start With the characters you entered.
In this example, if we search by the first name, it returns 3 results correct.
However, if we try to do a search by the last name, we will not get any results.
Change the Items property to allow Contains search rather than a StartsWith search.
If you would rather be able to search for values that contain the value you entered instead, you'll need to make some changes.
Select your field and in the Items property, change it this:
Filter(Choices([@'Demo List'].President), Not(IsBlank(Value)))
Search will now find anything that Contains the characters you entered.
Now, choices with any matching text will be returned in your results.