Summary
The behavior of lookup rules changed in Forms 9.1. By default, lookup rules will populate the matched field with possible values from the data source. In some cases, this behavior is not desirable.
History
In Forms 9.0, drop-down fields auto-suggested but single-line fields did not. In Forms 9.1, both drop-down and single-line fields auto-suggested. In Forms 9.1.1, you have the option to turn off the auto-suggest behavior by adding the aforementioned setting to the database.
Solution
On the product-level, you can disable the auto-suggest feature of Forms 9.1 lookup rules.
To disable the auto-suggest lookup rule behavior