Indexing is not done immediately. At least after selecting the next record, the index should get updated properly. Due to conversions, bugs or system crashes, the index may get corrupted. In this case find and lookup won't work properly. Importing in a clone of the database will force the construction of a new index.
Calculations take the indexed value for comparisons.
If you want to use an exact comparsion, you may use the Exact function. 4.2 Lookups Lookups are based on the indexed values of all records. It will copy a value from the first, matching record that it finds. This will cause a self lookup, pasting a matching value. This may even work on incomplete words.